FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()
Code now convinces browser to rebuild when the tree changes by UI.
When widgets are move, the current widget should always be visible.
It's the responsibility of the UI callback to update the browser.
tl;dr : making Fluid maintainable, no changes in code execution and logic.
This is a pretty extensive restructuring of the Fluid source tree.
It was neccessary because source and header files were getting
much too big to handle. Many source files had no header, and many
headers declared functions that were in diffrent source files.
Reorganized much of the include statements.
Added comments to some of the files.
Added Doxygen configuration file for standalone Fluid docs.
Tested everything by rebuilding Fluid .fl designs with the resorted
version of Fluid.
