win32: Use theme shipped with gtk

No reason for us to maintain own copy. This also
makes custom theme installing easier.
win32: Silent installer improvements

- Launch uninstaller as VERYSILENT
- Don't launch Python/Perl installers when silent
Update server list

Closes #1013
configure: Improve python logic

- add checks for python3.4
- only warn once for failure to find a version
- only run pkg-config call if the .pc file was actually found
- make unsupported python version non-fatal

Closes #1006
Closes #989
Disable tray support on Elementary

They followed Ubuntu in breaking any gtk trays..
travis: update before installing deps
Use GNUC format attribute on print functions

Closes #1059
Fix formatting warnings

Found by #1059
Merge branch 'master' of https://github.com/hexchat/hexchat