DrawText: Properly initialize and update in all cases.

There were several cases where DrawText would crash if the operations
are not in the right order.
Example:
DrawText myText;
myText.setText("Hello"); <- crash because font is not initialized

This commit also renames some methods of DrawText to be more consistent.
Window: Rename Update and Create methods to be more consistent with the existing naming scheme
Rename window.* to Window.* to be consistent with the file naming scheme

Files that define classes should begin with Uppercase
Merge various initializations into gomu::init(), and remove Window.

Rationale for removing Window:

In SDL 1.2, there can always be only one window at a time, rendering
more than one Window instance useless.
We also never need a reference to the underlying SDL_Surface*, because
we are rendering with OpenGL
Structure and build GomuEngine as a library