Improve deriving from Fl_Chart (STR 2022)

This commit makes the local, static drawing functions in the source
file available for subclasses by converting them to static protected
methods as requested by STR 2022.

The source and header files have been reformatted according to the
CMP, documentation for the new protected methods was added.

Source code *functionality* has not been changed.

This resolves and closes STR 2022.

Todo:
 - investigate whether these drawing methods wouldn't better be
   member functions and "convert" them if applicable
 - investigate whether the internal struct FL_CHART_ENTRY could become
   a local part of the Fl_Chart class, maybe Fl_Chart::Entry.
 - rename private member variables with trailing underscore (see CMP)

0 files changed, 0 insertions, 0 deletions