Preferences: Encourage to restart Gajim after switching themes

Until this change is handled right away to change every dependent
setting (notably the own message colour), this warning avoids users
wondering why colours are wrong.
2 jobs for ask-restart-theme in 7 minutes and 35 seconds (queued for 3 minutes and 38 seconds)
latest
Status Job ID Name Coverage
  Test
passed #21721
code-quality

00:05:23

3.0%
 
  Build
passed #21722
linux

00:02:11