Windows: Just for fun, allow the ability to hook into the System Menu

This means we can easily add an "About fpGUI" menu item in the System
Menu of a window.
docview: Adds an "About..." menu item in the Windows System Menu of the main form.

Just for fun and can be disabled if needed.
docview: updated green arrows - much less jaggies.
Merge branch 'develop' of https://github.com/graemeg/fpGUI into fpGUI-aros