[Plugins] Update create script and add GTK3 how-to doc

- Updated create_plugin script to create a GTK3 plugin.
- Added a document for updating a 1.3 plugin to be compatible with
  2.0.

docs/source/devguide/how-to/index.md

@@ -6,3 +6,7 @@ the tutorials.
 ## Web JSON-RPC
 
 - [Connect to JSON-RPC using curl](curl-jsonrpc.md)
+
+## Plugins
+
+- [Update 1.3 plugin for 2.0](update-1.3-plugin.md)

docs/source/devguide/how-to/update-1.3-plugin.md

@@ -0,0 +1,132 @@
+# How to update a Deluge 1.3 plugin for 2.0
+
+With the new code in Deluge 2.0 there are changes that require authors of
+existing plugins to update their plugins to work on Deluge 2.0.
+
+The main changes are with Python 3 support and the new GTK3 user interface with
+the dropping of GTK2. However it is still possible for a 1.3 plugin to be made
+compatible with 2.0 and this guide aims to helps with that process.
+
+Note that the Deluge 2.0 plugins now use namespace packaging which is not
+compatible with Deluge 1.3.
+
+## Python
+
+### Python version matching
+
+Ensure your code is both Python 2.7 and Python >=3.5 compatible.
+
+In `1.3-stable` the plugins that were built with a specfific version of Python
+could on be loaded if the system Python also matched.
+
+This has change in Deluge 2.0 and it will load any Python version of plugin
+eggs so compatibility is essential for end-users not to encounter issues.
+
+### Six
+
+Use [six] to assist with compatibility.
+
+[six]: https://pythonhosted.org/six/
+
+### Unicode literals
+
+Add this to files to ensure strings and bytes separatation so there are no
+surprises when running on Python 3.
+
+## GTK 3 addition
+
+In order to support both Deluge 1.3 and 2.0 all existing plugin GTK UI files
+must be copied and then converted to contain only GTK3 code with the old files
+still using PyGTK e.g.:
+
+    cp gtkui.py gtk3ui.py
+
+### Convert from libglade to GtkBuilder
+
+With PyGTK there were two library options for creating the user interface from
+XML files by default Deluge plugins used libglade but that has been deprecated
+and removed in GTK3. So the libglade `.glade` files will need converted to
+GtkBuilder `.ui` files and the Python code updated.
+
+https://developer.gnome.org/gtk2/stable/gtk-migrating-GtkBuilder.html
+
+#### gtk-builder-convert
+
+Install the `gtk-builder-convert` converter on Ubuntu with:
+
+    sudo apt install libgtk2.0-dev
+
+To convert your GTK run it like so:
+
+    gtk-builder-convert data/config.glade data/config.ui
+
+#### Glade UI designer for GTK2
+
+The above conversion can be done in Glade UI designer (version <=3.8), ensuring
+that the minimum Gtk version is set to 2.24 and any deprecated widgets are
+fixed. The updated file should be saved with file extension `.ui`.
+
+#### Python code changes
+
+The code needs to replace `gtk.glade` references with `gtk.Builder` and the
+first step is updating how the files are loaded:
+
+```diff
+- self.glade = gtk.glade.XML(get_resource("config.glade"))
++ self.builder = gtk.Builder()
++ self.builder.add_from_file(get_resource("config.ui"))
+```
+
+The next stage is to replace every occurange of these `glade` methods with
+the `builder` equivalents:
+
+    glade.signal_autoconnect -> builder.connect_signals
+    glade.get_widget -> builder.get_object
+
+### Migrate XML files to GTK3
+
+If you open and save the file it will update with the new requirement header:
+
+    <!-- Generated with glade 3.18.3 -->
+    <interface>
+      <requires lib="gtk+" version="3.0"/>
+
+You can fix deprecated widgets but keep the minimum GTK version to <= 3.10 for
+desktop compatiblity.
+
+An example of migrating a Deluge plugin to GtkBuilder: [AutoAdd GtkBuilder]
+
+### Gtk import rename
+
+Move from PyGTK to GTK3 using Python bindings.
+
+https://pygobject.readthedocs.io/en/latest/guide/porting.html
+
+    wget https://gitlab.gnome.org/GNOME/pygobject/raw/master/tools/pygi-convert.sh
+    cp gtkui.py gtk3ui.py
+    sh pygi-convert.sh gtk3ui.py
+
+```diff
+-import gtk
++from gi.repository import Gtk
+```
+
+```diff
+- self.builder = gtk.Builder()
++ self.builder = Gtk.Builder()
+```
+
+### Deluge GTK3
+
+Imports will need updated from `deluge.ui.gtkui` to `deluge.ui.gtk3`.
+
+### PluginBase
+
+```diff
+-from deluge.plugins.pluginbase import GtkPluginBase
++from deluge.plugins.pluginbase import Gtk3PluginBase
+-class GtkUI(GtkPluginBase):
++class Gtk3UI(Gtk3PluginBase):
+```
+
+[autoadd gtkbuilder]: https://git.deluge-torrent.org/deluge/commit/?h=develop&id=510a8b50b213cab804d693a5f122f9c0d9dd1fb3