WIP: Text

WIP. Before merge, need to add tests.

Currently working:

1. Calculation (Cython) and utilization of SDF.

2. Linux text display and system font listing.

3. OSX text display and system font listing.

4. Win32 text display and system font listing.

5. Automatic downloading of a couple nice open-source fonts.

6. Make it a `Visual`.
With SDF all mini-textures have equal size, so packing them together can be done much easier. You can basically specify the size of the sub-textures at the instantiation of the Atlas.