Theming: theme change notification

[Dave Rabelink]

I would like to see a new SAM message, for instance :

SAM_ThemeChanged

when the TD theme was changed.

When the GUI is changed to another theme, we could need to recolor/repopulate/repaint objects accordingly.
For instance, icons or bitmaps placed in the GUI could look out of place in a newly set theme.
Or maybe custom drawing should adapt other colors to fit in the new theme.

Having such a notification automatically on every top level window we are able to call custom functions to change all those objects to more appropriate colors.

(the requirement to get the base theme colors can be used to change the current colors of objects to use the colors of the newly set theme)