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>
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
Improved the detection of whether a layer is clicked or not
+ Implemented detection for layer clicking for the ellipses (open and filled)
* Simplified the code for detecting whether a rectangle was clicked
* Removed duplicate code from LayerPath

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Some minor improvements on the GUI

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
An error message is now displayed when an error occurs while saving or loading a file

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Improved the file loading and saving mechanism
+ The user is now warned if there are unsaved changes
* Loading a file now clears the history (as it should do)

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