region: Change "Input Sources" label

Use "Keyboard Layout" instead.

Note 2020-08-26: This change was made in Endless to make the UI more
  clear, so I'm proposing it upstream. - mwleeds
8 jobs for mwleeds/rename-input-sources in 7 minutes and 25 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #873089
build

00:04:21

failed #872853
build

00:00:44

 
  Test
passed #872854
test

00:03:03

 
  Manual
manual #872856
privileged allowed to fail manual
asan
manual #872855
allowed to fail manual
flatpak
manual #872859
allowed to fail manual
msan
manual #872857
allowed to fail manual
tsan
manual #872858
allowed to fail manual
ubsan