Zeta GUI library overview

Zeta GUI library overview

The Zeta GUI library will be a featherweight GUI library written in
C++. It's main design goals will be speed, flexibility, and
portability, using the SDL, Allegro, or other graphics libraries for
video output.

It will support multiple windows, and within each window, widgets.
However, the user will have to define the "root" window coordinates,
or specify to use the entire screen. So, Zeta will be able to be used
as a full GUI library, or as a partial GUI library.

Positioning will be fairly simple, simply percentages of the parent
window or percentages of the root window. As the root window is
resized, widgets will be rescaled on-demand.

Zeta support loading data from .XML files (perhaps using libexpat?). Themes
will be also be supported eventually.

This commit is a basic outline of the headers, still very much incomplete, but it should give a basic idea of how the library-indepentent code will work.