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`.
If we chose to use SDF font rendering, we don't need this (or at least can do with a much simpler version).