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.

If FLTK has been built with bundled image libs the include directory
of the *installed* library headers is 'FL/images' relative to the
normal 'include' directory.

The CMake property "include_directories" is now added to the installed
fltk_images library so it is used when building user programs with
bundled image libs.

This commit adds support of scenarios where a child window is moved while its parent is simultaneously modified.

Adjust internal vector sizes before and after changing the rows() in
the base class to avoid inconsisten sizes.

Note that the crash reported by the OP of PR #1187 was caused by
more than one issue, not only the one fixed here. Thanks for the
original patch to the author, GitHub user 'basiliscos'.

Users who need the old behavior for strict backwards compatibility under Windows can load the previous (Arial) font at program startup with only a few lines of code:

#ifdef _WIN32
  // reset Windows fonts to pre-1.4.2 state
  Fl::set_font(FL_HELVETICA,     " Arial");
  Fl::set_font(FL_HELVETICA + 1, "BArial");
  Fl::set_font(FL_HELVETICA + 2, "IArial");
  Fl::set_font(FL_HELVETICA + 3, "PArial");
#endif

Fl_Window::current() can refer to a deleted window is some scenarios

1. prepend include dirs of bundled image libs and append Cairo include
   dirs and flags

2. remove forgotten debug statement from previous commit

Avoid adding a non-existent folder to include directories (-I)
in `fltk-config --cxxflags` etc.

This is a safety for consistency. It doesn't really change the result
but avoids "adding" an empty argument to a CMake list.

On FreeBSD some of the required header files (and libraries) are not
in standard (known) locations, hence they wouldn't be found under
certain circumstances. Making all dependencies 'PUBLIC' adds the
needed include directories and link libraries to all builds.

For details see GitHub Issue #1173 with title
  "FLUID compilation error on FreeBSD when FLTK_BACKEND_X11=ON"

This makes the IM aware of the cursor movement to the right after having committed text.