Add/finish the text/font metrics system

[boydm]

It would be useful for the scene to be able to obtain the metrics (width, height, etc) of a string of text before it gets rendered. For example, the Button component could use this to automatically size the button to fit the string. Any future (optional) layout engine would also need it.

This should NOT parse fonts at runtime. Interpreting fonts should remain the purview of the Drivers. In the future I want to support other font types than *.ttf and the core Scenic library should not have a dependency on that format.

Instead, I want to back the font metric information into separate format offline that can be easily loaded and inspected by a scene. No C code should be invoked at that time. This work is under way, but has been lower priority as most (all) scenes so far simply don't need it.

[boydm]

Now in master. Going out in v.10