ENH: Text

Currently working:

1. Calculation (in shaders) and utilization of SDF.

2. Linux text display and system font listing.

3. OSX text display and system font listing.

4. Win32 text display (auto-download of compiled 32- and 64-bit freetype libs) and system font listing.

5. Automatic downloading of a couple nice open-source fonts.

6. Make it a `Visual`.

7. Very basic testing, we can improve once the visual-testing PR is merged.