* define the pen/tablet support API
* add pen event handler stub as a fallback
* add pen device test "penpal".
* Add macOS pen/stylus/tablet driver.
* Add Oxygen documentation.
* Change class Fl into namespace Fl.
* Untangle Fl namespace into themed headers.
* cut line count of FL/Fl.H in half
* FL/core subdirectory now holds short headers
grouped by functionality.
