combo-row: Add the use-subtitle property

Allow to display the current value as the subtitle rather than at the
end of the row.

Fixes Librem5/libhandy#95
7 jobs for combo-row-subtitle in 13 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #48466
librem5
build-debian-clang

00:02:35

passed #48464
librem5
build-debian-gcc

00:02:55

passed #48465
librem5
build-debian-gcc-static

00:02:01

passed #48467
librem5
build-fedora-gcc

00:05:11

 
  Docs
passed #48468
librem5
build-gtkdoc

00:01:32

 
  Test
passed #48469
librem5
unit-test

00:02:52

46.0%
 
  Package
passed #48470
librem5
package-debian

00:02:03