region: Tidy up GtkWidget variables

- Use correct types for each widget
- Put variables together in alphabetical order in the class
- Drop unused IDs
- Rename .ui file to match .c file
11 jobs for <span class="ref-name">region-gtkwidget-types</span> in 5 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #122542
build

00:01:58

 
  Test
passed #122544
asan

00:02:10

passed #122543
test

00:01:47

 
  Manual
manual #122549
allowed to fail manual
armv8
manual #122545
allowed to fail manual
flatpak
manual #122548
allowed to fail manual
msan
manual #122550
allowed to fail manual
ppc64le
manual #122546
allowed to fail manual
tsan
manual #122547
allowed to fail manual
ubsan
 
  Review
manual #122551
allowed to fail manual
review
manual #122552
allowed to fail manual
stop_review