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.

FLTK now supports 2 types of fullscreen modes under macOS:
1) normal macOS fullscreen mode where the menubar and titlebar appear when mouse is moved to top of screen
and the window covers one screen only;
2) multiple-screen mode that requires calling Fl_Window::fullscreen_screens() before.

This commit fixes all possible transitions between these fullscreen modes and normal window mode.

Separate both constructor types and document parameters.