GtkEntryCompletion: position using gdk_window_move_to_rect()
In GNOME Weather we have an issue gnome-weather#187 about a case when it's impossible to select a location on external display. It turns out that the bug is in GTK. There are already two unmerged MRs trying to fix the bug. This MR is a continuation of the work started by @mistmist in !3487. It further simplifies popup resizing logic and disconnects moved-to-rect
callback.
Fixes: #2794