A barebones GUI library for Win32 and X11.

This is a fork of nakst's luigi library, but with changes that may not be acceptable to that library. The major changes are:

• No longer single header.
• Built using CMake.
• Removal of Essence port (CMake does not support Essence).

There are also slight changes that make this no longer compatible with nakst's version. The biggest change is making the ui context variable opaque. Accessing properties of this structure are done via some new functions. Please see example apps.