HdyActionRow: set the subtitle as a tooltip

Currently, if the subtitle is long enough the user can't just hover the subtitle to read it's content and have to resize the window which is not great
7 jobs for hdy-action-row/subtitle in 11 minutes and 3 seconds
latest
Status Job ID Name Coverage
  Build
passed #56850
librem5
build-debian-clang

00:02:47

passed #56848
librem5
build-debian-gcc

00:03:01

passed #56849
librem5
build-debian-gcc-static

00:02:04

passed #56851
librem5
build-fedora-gcc

00:02:47

 
  Docs
passed #56852
librem5
build-gtkdoc

00:02:00

 
  Test
passed #56853
librem5
unit-test

00:02:54

46.5%
 
  Package
passed #56854
librem5
package-debian

00:03:07