Added the open and filled circle tools
Drawings can now be saved
+ Added methods in the Layer classes to save them to XML
+ Added a method in the Schets class to save it to XML
+ Added menu items to save and load drawings

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Drawings can now be loaded
+ Added methods to the Layer classes to load from a XML document
+ Added a method to Schets to load from a file

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added gum tool, IsClicked methods for all layers except ellipses and 'removed' SchetsAction
Merged Cas' work

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Fixed some bugs
* The eraser tool no longer inherits from the pen tool (so no longer a line is drawn when holding down the eraser tool)
* The Draw() method of PathLayer was broken
* The eraser tool now erases the top layer
* When erasing a layer is undone, it's placed at its original index in the layer list
* A tool now gets notified when there is being changed to another tool