texthandles: Use GtkWindow popovers API
texthandles: Use GtkWindow popovers API

This offers the same behavior, but GDK_WINDOW_TEMP windows aren't used
anymore, involving less translations from/to root coordinates, plus no
glitches in having handles snap to content as windows move.

davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,Lyude/gtk-,alexlarsson/gtk,ahodesuka/gtk,alexlarsson/gtk,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,chergert/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,jessevdk/gtk,Lyude/gtk-,ahodesuka/gtk,Lyude/gtk-,jigpu/gtk,msteinert/gtk,chergert/gtk,ahodesuka/gtk,jessevdk/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,ahodesuka/gtk,chergert/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,msteinert/gtk,ahodesuka/gtk,ahodesuka/gtk,jessevdk/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Lyude/gtk-,Adamovskiy/gtk,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,chergert/gtk,Lyude/gtk-,jigpu/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,jessevdk/gtk,msteinert/gtk,jessevdk/gtk,msteinert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,davidgumberg/gtk,msteinert/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,msteinert/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,jigpu/gtk,chergert/gtk,ahodesuka/gtk,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,jessevdk/gtk,alexlarsson/gtk