GTK: cache the completion popup window
GTK: cache the completion popup window

This avoids creating and destroying windows quickly under stress, which
may lead to XID collisions.

See e.g. https://bugzilla.gnome.org/show_bug.cgi?id=581526 and
https://bugzilla.gnome.org/show_bug.cgi?id=590690

Closes [bugs:1649].

R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone,R1dO/scintilla_clone