- remove configure stuff from more README files
- move documentation/README.txt to README.documentation.txt

This removes the last instance in FLTK of use of #if __APPLE__ instead of the driver mechanism.

- remove configure related stuff
- improve documentation in these files

More to come...

- remove autotools build (build-autotools)
- build documentation with `cmake`, not `make`

To do: build fluid docs

New functions append "_str" to the function name to
avoid ambiguities when calling them. So
'char *fl_filename_name(const char *)' becomes
'std::string fl_filename_name_str(const std::string &)'

One file per logical unit.
Namespaces.
Non-static data member initializers to never
get an uninitialized field again.

... so the user can override all settings.

This may need more tweaks in the future.

libXft was erroneously present in the link command when using Pango and Cairo.
This is fixed by disconnecting the GUI scaling code from use of Xft.

This commit also makes sure that when Wayland is used, pkg-config is available
on the build machine. This allows to remove from file CMake/options.cmake
code that was labelled with "FIXME".

Still to do: rename fl_filename... to fl_filename..._str
and introduce into core library.

Also fix for Linux.

All mentions of Fl_String will be replaced with std::string
and the Fl_String implementation will be removed.

FLTK 1.5 requires at least C++11, hence we can always use 'override'.

To do: the FL_OVERRIDE macro can be replaced by 'override' everywhere
in the FLTK code. There are more than 2400 occurrences and this
would be a large commit. Therefore it's not done right now...

Note: user code may continue to use FL_OVERRIDE.

Note: this is only a hidden implementation detail: all concerned
variables are private, and the code is simplified (less error prone).

Size of Fl_Group on 64-bit Linux: 168 -> 176 Bytes (+8 Bytes).

test/group.cxx: test for Fl_Group::{add, insert, remove} etc.

Note: current minimum for future 1.5.0 is C++11, this may be raised
  after further discussion.

This code was used in the past to support a deprecated way to obtain the initial
value of the GUI scaling factor for the X11 platform.

make remaining casts consistent with other casts of the same
variables

Note: ubuntu-20.04 will be phased out soon