meldapp: Add 'support' for launching non-uniquely, which is our default
meldapp: Hold the command line instead of disabling single instance
meldapp: Support new windows owned by the single instance
Focus the first tab opened from a new-tab command line
meldapp: Integrate with OptionParser and output to local stdout

This fixes some issues with new tab/window creation and lifecycles,
as well as introducing a godawful hack to integrate GApplication
and GApplicationCommandLine with optparse. Without support for local
command line handling this is never going to be pretty.

There are several remaining slightly weird issues here, but it works
and most people hopefully won't hit the edge cases. Better support
is coming in glib 2.40 for doing this sort of thing, so let's hope that
pans out.
meldapp: Remove now-inaccurate comment
meldapp: Add fallback for when the print_literal hack inevitably breaks
meldapp: PEP8