Edit/Special Characters... does not work - selecting characters has no effect.
Fixing this looks like a weekend hacking project on gtk-osx. As far as I can tell from looking at documentation, the character palette works via NSInputManager, and when the user selects a character, the NSView with the keyboard focus will get an insertText message.
To make this work in GTK+, we have to get that insertText message into the GTK+ input method framework - there needs to be a GtkIMContextQuartz that hooks into the insertText method on GdkQuartzView and turns that into a ::commit signal. Note that for our purposes here I don't think we need an implementation of NSTextInput. (To fully implement that would require extending GtkIMContext with additional signals. To partially implement it would require testing with different input methods to see whether a partial implementation is useful or just causes things to break.)
