GtkEntryCompletion: position using gdk_window_move_to_rect()

Vitaly Dyachkovrequested to merge
obyknovenius/gtk:bug-2794 into gtk-3-24

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

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