region: Use a GtkPopover instead of GtkDialog for input options

As per design:
https://wiki.gnome.org/Design/SystemSettings/RegionAndLanguage
11 jobs for <span class="ref-name">region-input-options-popover</span> in 3 minutes and 42 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #104219
build

00:02:07

 
  Test
passed #104221
asan

00:01:34

passed #104220
test

00:00:37

 
  Manual
manual #104226
allowed to fail manual
armv8
manual #104222
allowed to fail manual
flatpak
manual #104225
allowed to fail manual
msan
manual #104227
allowed to fail manual
ppc64le
manual #104223
allowed to fail manual
tsan
manual #104224
allowed to fail manual
ubsan
 
  Review
manual #104228
allowed to fail manual
review
manual #104229
allowed to fail manual
stop_review