Merged requested to merge alxsa-backport-resize-canvas-templates into gimp-2-10
This is a backport of @StanislavGrinkov's solution to #1140 (closed) via !397 (merged) and !406 (merged).
The code is nearly identical. The only differences are one GTK3 -> GTK2 change (
gtk_radiobutton_join_group () to
gtk_radiobutton_set_group ()) and UI re-arrangements to match the new dialog layout backported in !821 (merged).