[Lazarus] TArrow, help for graph primitives needed

[Juha Manninen]

Regarding issue:
21117: Can not change background Color of TArrow
  http://bugs.freepascal.org/view.php?id=21117

I tested the patch from Tomasz Wieckowski and it works.
The improvement affected only Windows and I wanted to turn it into a
cross-platform solution.
It seemed easy because the used graph functions are already cross-platform.
FillRect and FillRgn are these functions.

I made the code as a widgetset default implementation, TWSArrow.
You must compile Lazarus with "NewArrow" define to test it.
With "NewArrow" all widgetsets use this default implementation.
Earlier there was specialized implementation for Windows, WinCE and
QT.

Unfortunately it only works on Windows. FillRgn does not work with
GTK2 and it crashes with QT. Apparently it is not properly
implemented.
Could somebody graphics oriented please look at TWSArrow and maybe fix it.

Later the code should move out from widgetset bindings. None of the
widgetsets have a native "arrow" widget. The code always used graphics
primitives to draw it.

Juha