Run GTK main loop in between incoming stanzas

- This prevents 30-60 second lockups when starting gajim while the main thread
  is processing large numbers of incoming stanzas and not yielding to the GTK
  main loop at all.
- This is done by explicitly running the main loop after a stanza is received,
  if there are events pending.

(Empirically, this makes gajim actually usable on my machine, whereas current
master is pretty bad lockup-wise after startup/reconnection...)
3 jobs for eta/gotta-go-fast in 0 seconds (queued for 62 minutes and 1 second)
latest
Status Job ID Name Coverage
  Test
failed #18114
run-appdata

failed #18113
run-tests

 
  Build
skipped #18115
run-build
 
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failed
run-tests Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
run-appdata Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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