Changes to file loading and saving

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

I've just pushed a merge to master, transitioning all of our file
loading and saving (in file comparisons only) to use GtkSourceView's
new-as-of 3.14 file helpers.

Aside from gaining vastly improved performance and async loading &
saving, I'm hoping that this will have fixed several annoyances around
varying end-of-line issues as well as a few problems with file
encoding. As this is a pretty big change, there's a fair chance that
we'll hit some regressions, so please don't be shy about pointing out
any you come across.

It's worth noting that if you currently have additional text encodings
configured, you'll need to have GtkSourceView 3.18 installed for those
to be used. If you're on 3.14 or 3.16, we'll only use the default list
of encodings that GtkSourceView provides for your locale. However,
it's also worth noting that the list of encodings that GtkSourceView
tries by default is much more liberal than Meld's old defaults.

cheers,
Kai