Bring up-to-date with changes to GTType.
A big commit adding support for custom image managers.

The idea behind this system is to allow the host application or library
to define it's own image loading routines. As a result, GTGUI no longer
depends on GTImage. If, for example, the host wants to use DevIL
instead of GTImage, it can now do so.
Unset the glyph map manager of the font server in the GTGUI::Server destructor.