Use xvfb on travis

As instructed on http://docs.travis-ci.com/user/gui-and-headless-browsers/
Disable some failing tests

These tests may no longer be correct. Their author is looking into them.
Merge branch '1.12' of github.com:wesnoth/wesnoth into 1.12
Always indicate that faction was chosen when placing player.

This fixes bug #21977.
Minor style improvements.