Use msys2 for the AppVeyor build

[rnestler]

I'm opening this to ask if it would be appreciated if I would change appveyor.yml to use msys2 to do the builds.
Advantages:

• The official guide to build on Windows recommends using msys2
• Probably simpler build script

Disadvantages:

• Only one GTK version supported

[gkoz]

I assume you're talking about getting libgtk+ from msys2 and not mingw -- I've not managed to achieve successful build with that mingw (#218 (comment)).

Since our building instructions now suggest getting gtk from msys2, it's reasonable to actually test that. I wouldn't want to drop the current 3.14 variant just yet.

[rnestler]

Yes I'm talking about getting libgtk+ from the msys2 package. I put together a appveyor.yml that builds successfully: https://ci.appveyor.com/project/rnestler/gtk/build/1.0.3/job/6hk358eoero63gbq