Bug fix: Initialize PlatformSettings within SystemFonts

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

* Re-declare PlatformSettings::initialized under public scope --
was private -- so that SystemFonts::initialize can access the method.
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.