melddoc: Update open-external action to use GTK helper (bgo#698518)

Gtk.show_uri() now appears to do the right thing as far as focus
stealing prevention goes, so we'll use that instead of launching
xdg-open.
misc: Fix traceback when showing a parentless error dialog
meldapp: Behaviour fix for exiting on parser errors (bgo#730392)

Because of the remote instance handling weirdness with the OptionParser
handling, we need to try and handle parser errors in multiple places,
with different handling for internal OptionParser error calls vs. our
own error handling.
meldapp: Improve some error case logic when creating tabs
meldapp: Actually present the window for correct raising behaviour
meldapp: Don't try to set tab extras on failed tabs
meldapp: When tab creation errors out, remove any added window
Updated Polish translation
Updated Russian translation
WIP: Port to C++