Drop 1.0 compatibility; document 1.4 migration

The FLTK 1.0 compatibility support by defining macro 'FLTK_1_0_COMPAT'
did no longer work since FLTK 1.3.x and nobody complained. Hence we
can safely remove this "feature".

Documentation changes:

 - Drop chapter 'Migrating Code from FLTK 1.0 to 1.1'
 - Drop chapter 'Migrating Code from FLTK 1.1 to 1.3'
 - Add  chapter 'Migrating Code from FLTK 1.3 to 1.4'

The removed chapters are still available in FLTK 1.3 docs.