Create a plan and an estimation for an eraser within annotation functionalities

[mkempka]

• The PDF widget should get an eraser
• The canvas for drawing on images should get an eraser

[jkrause]

There are plenty of challenges implementing an eraser:

• Dividng existing splines into multiple parts when erasing
• Erasing a part of a spline in the way that the spline gets thinner
• Undo / Redo integration