app: Remove old GimpUiManager popup API

The code used `gtk_menu_popup()`, which was deprecated a while ago, as
it was broken on certain GDK backends, like Wayland.

Fixes #5319
7 jobs for !526 with nielsdg/fix-context-menus in 32 minutes and 57 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Prepare
passed build-image #1662331

00:02:04

passed build-image-win64 #1662332

00:01:31

 
  Dependencies
passed deps-debian #1662333

00:01:52

passed deps-win64 #1662334

00:10:33

 
  Gimp
passed gimp-distcheck-debian #1662335

00:28:50

passed gimp-meson-debian #1662336

00:05:28

passed gimp-win64 #1662337

00:11:02