workspace: Learn sticky.

A floating window that's sticky will move to the new active workspace
whenever the workspace on the same output changes.

include/container.h

@@ -74,6 +74,7 @@ struct sway_container {
 	bool visible;
 	bool is_floating;
 	bool is_focused;
+	bool sticky; // floating view always visible on its output
 
 	// Attributes that mostly views have.
 	char *name;