The idle callback removal clause in the ScintillaGTK destructor got
broken in the last Scintilla update [1], leading to the callback not
being removed thus possibly running after the instance destruction.
Indeed, gdk_threads_add_idle() wraps g_idle_add() with a custom user
data, thus making the g_source_remove_by_user_data() call in the
destructor incorrect, as we give it our own user data, not GDK's
wrapper one.
Fix the callback removal not to use user data matching to avoid this.
Closes#1033.
[1] 4e5c321dda/
X-Scintilla-Bug-URL: https://sourceforge.net/p/scintilla/bugs/1827/
X-Scintilla-Commit-ID: d889200cd9de032e278745f48b9c3108ccfa5984
cb8151b29b