Add shortcut to save snapshot of current page

Adds a shortcut CTRL+F9 to save the current page as png file.

Fixes: GNOME/epiphany#259
3 jobs for <span class="ref-name">save-page-snapshots</span> in 4 minutes and 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #252674
flatpak

00:03:16

 
  Deploy
passed #252675
review

00:01:00

manual #252676
allowed to fail manual
stop_review