Port to gedit 3.12

GEdit version 3.12 changed the API for plugins; in particular, this patch
interfaces correctly with the side panel, which is now a GtkStack.
Try to load symbols on draw event

We must make sure that symbols are loaded for current document even
if the sourcetree does not get the focus: it is sufficient that it
is drawn.
Fix copying of remote files with Python 3

This closes LP #1313112
Replace the deprecated Gtk.VBox with Gtk.Box
Merge pull request #1 from toobaz/mojave_3_12

Support for 3.12