Implement Tray Icons

This commit implements the StatusNotifierItem protocol, and enables
swaybar to show tray icons. It also uses `xembedsniproxy` in order to
communicate with xembed applications.
The tray is completely optional, and can be disabled on compile time
with the `enable-tray` option. Or on runtime with the bar config option
`tray_output none`.

Overview of changes:
In swaybar very little is changed outside the tray subfolder except
that all events are now polled in `event_loop.c`, this creates no
functional difference.

Six bar configuration options were added, these are detailed in
sway-bar(5)

The tray subfolder is where all protocol implementation takes place and
is organised as follows:

tray/sni_watcher.c:
	This file contains the StatusNotifierWatcher. It keeps track of
	items and hosts and reports when they come or go.
tray/tray.c
	This file contains the StatusNotifierHost. It keeps track of
	sway's version of the items and represents the tray itself.
tray/sni.c
	This file contains the StatusNotifierItem struct and all
	communication with individual items.
tray/icon.c
	This file implements the icon theme protocol. It allows for
	finding icons by name, rather than by pixmap.
tray/dbus.c
	This file allows for asynchronous DBus communication.

See #986 #343

swaybar/render.c

@@ -8,6 +8,10 @@
 #include "swaybar/config.h"
 #include "swaybar/status_line.h"
 #include "swaybar/render.h"
+#ifdef ENABLE_TRAY
+#include "swaybar/tray/tray.h"
+#include "swaybar/tray/sni.h"
+#endif
 #include "log.h"
 
 
@@ -297,6 +301,72 @@ void render(struct output *output, struct config *config, struct status_line *li
 	}
 	cairo_paint(cairo);
 
+#ifdef ENABLE_TRAY
+	// Tray icons
+	uint32_t tray_padding = config->tray_padding;
+	unsigned int tray_width = window->width * window->scale;
+	const int item_size = (window->height * window->scale) - (2 * tray_padding);
+
+	if (item_size < 0) {
+		// Can't render items if the padding is too large
+		goto no_tray;
+	}
+
+	if (config->tray_output && strcmp(config->tray_output, output->name) != 0) {
+		goto no_tray;
+	}
+
+	for (int i = 0; i < tray->items->length; ++i) {
+		struct StatusNotifierItem *item =
+			tray->items->items[i];
+		if (!item->image) {
+			continue;
+		}
+
+		struct sni_icon_ref *render_item = NULL;
+		int j;
+		for (j = i; j < output->items->length; ++j) {
+			struct sni_icon_ref *ref =
+				output->items->items[j];
+			if (ref->ref == item) {
+				render_item = ref;
+				break;
+			} else {
+				sni_icon_ref_free(ref);
+				list_del(output->items, j);
+			}
+		}
+
+		if (!render_item) {
+			render_item = sni_icon_ref_create(item, item_size);
+			list_add(output->items, render_item);
+		} else if (item->dirty) {
+			// item needs re-render
+			sni_icon_ref_free(render_item);
+			output->items->items[j] = render_item =
+				sni_icon_ref_create(item, item_size);
+		}
+
+		tray_width -= tray_padding;
+		tray_width -= item_size;
+
+		cairo_set_source_surface(cairo, render_item->icon, tray_width, tray_padding);
+		cairo_rectangle(cairo, tray_width, tray_padding, item_size, item_size);
+		cairo_fill(cairo);
+
+		item->dirty = false;
+	}
+
+
+	if (tray_width != window->width * window->scale) {
+		tray_width -= tray_padding;
+	}
+
+no_tray:
+#else
+	const int tray_width = window->width * window->scale;
+#endif
+
 	// Command output
 	if (is_focused) {
 		cairo_set_source_u32(cairo, config->colors.focused_statusline);
@@ -309,12 +379,11 @@ void render(struct output *output, struct config *config, struct status_line *li
 	if (line->protocol == TEXT) {
 		get_text_size(window->cairo, window->font, &width, &height,
 				window->scale, config->pango_markup, "%s", line->text_line);
-		cairo_move_to(cairo, (window->width * window->scale)
-				- margin - width, margin);
+		cairo_move_to(cairo, tray_width - margin - width, margin);
 		pango_printf(window->cairo, window->font, window->scale,
 				config->pango_markup, "%s", line->text_line);
 	} else if (line->protocol == I3BAR && line->block_line) {
-		double pos = (window->width * window->scale) - 0.5;
+		double pos = tray_width - 0.5;
 		bool edge = true;
 		for (i = line->block_line->length - 1; i >= 0; --i) {
 			struct status_block *block = line->block_line->items[i];