Added basic undo/redo support
+ Added the class SchetsAction that can be derived from to represent actions that are committed to the drawing
+ Added members to SchetsControl to manage the action history
+ Added undo and redo buttons to SchetsWin
* Modified the Tool-classes to commit the action they do
* Tools are now only used with the left mouse button
- Removed the unnecessary method veranderAfmeting() from SchetsControl (and also from Schets)

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>