app: Allow setting telepathy-glib debug flags

Sigh, this really should be provided out-of-the-box ...

GNOME/polari#144
3 jobs for !159 with tp-debug in 7 minutes and 42 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Review
passed check_commit_log #747594

00:00:28

passed eslint #747595

00:00:33

 
  Build
passed flatpak #747596
flatpak

00:07:08