wacom: Update "Test your settings" button sensitivity on device availability

The button/popover are meaningless if there's no device to test with. Set
it inactive (so the popover hides if visible) and set insensitive if no
devices are found.
11 jobs from wip/carlosg/wacom-test-button-sensitivity in 4 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #212710
build

00:01:36

 
  Test
passed #212712
asan

00:02:50

passed #212711
test

00:00:49

 
  Manual
manual #212717
allowed to fail manual
armv8
manual #212713
allowed to fail manual
flatpak
manual #212716
allowed to fail manual
msan
manual #212718
allowed to fail manual
ppc64le
manual #212714
allowed to fail manual
tsan
manual #212715
allowed to fail manual
ubsan
 
  Review
manual #212719
allowed to fail manual
review
manual #212720
allowed to fail manual
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