Bug fix: Initialize PlatformSettings within SDLApp

This prevents the font example from crashing at initialization, among
any other that wasn't using any functionality from the GUI engine.
Introduce font kerning for TrueType fonts

This feature only works with a patched version of SDL2_ttf. See the
bug report filled at https://bugzilla.libsdl.org/show_bug.cgi?id=2572
for details.
.travis.yml: Fix tar extraction of deps
travis-ci: Fix downloading of deps & update url