st/icon: Add a default fallback icon

The idea behind commit 3dd8ffc2 to try harder to avoid empty
icon actors because of missing icons was sound, so implement that
behavior in StIcon itself:

If the main gicon was not found, and the fallback gicon isn't set or
wasn't found either, fall back to the standard 'missing-image' icon.

GNOME/gnome-shell!1127
8 jobs for !1127 with fix-fallback-gicon in 6 minutes and 34 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #641236
check_commit_log

00:00:27

passed #641238
eslint

00:01:04

passed #641237
js_check

00:00:33

passed #641240
no_template_check

00:00:28

passed #641239
potfile_check

00:00:34

 
  Build
passed #641241
build

00:04:17

 
  Test
passed #641242
test

00:01:06

passed #641243
test-pot

00:01:11