Added the possibility to reorder the layers

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added anti-aliasing to make the graphics nice and smooth

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>
Added a help window
+ Added HelpWindow which contains a help window
+ Added a menu entry 'Help' to the 'Help' menu of Hoofdscherm to open the help window
* When importing a bitmap it is now just added as a layer (no longer are the other layers and history removed)
* Created a helper method setTool() in SchetsWin to prevent duplicate code
* A filled rectangle is now drawn without anti-aliasing
* Some minor code and (mostly) comment improvements

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