This adds proper Unicode support by allowing the parse system to read
UTF-8 encoded files. The new mode is controlled by a mod option which
allows older mods to continue using the legacy text encoding.
That option changes the behavior of several systems in FSO. It disallows
the usage of Volition fonts since they don't support Unicode encoding
and changes a few usages of special characters to their unicode
counterparts of generic rendering solutions.
This fixes#1415.
