Clipping and Masking should each have their own project page

[rhauck]

[rhauck]

@dirkschulze - do you have any objections to splitting these into 2 separate pages and cross-linking where appropriate?

(this would solve #85, too)

[dirkschulze]

Yes, it could probably be split. Would need several modifications though. I could try to do it in 1-2 weeks if no one takes it up.