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

Note: this fixes the warning, but a better fix would be to use either
  strlcpy(), fl_strlcpy(), or a string type in 1.5.0 or higher.

... in source file and documentation

This change allows to capture the rounded corners of a top-level GL window.

Also, rename Fl_Cocoa_Window_Driver::capture_decorated_window_10_5()
from capture_decorated_window_10_6() because this function uses
CGWindowListCreateImageFromArray() available from macOS 10.5.

Also replace 'MSWindows' with 'Windows' where appropriate, leaving
only old documents like README files from 1.3 and older as-is.

Also, use more explicit button names

This commit adds only documentation.