adding formats

[jlnalber]

As of now, you have endless space on the whiteboard. But for writing purpose, it often is useful to only have one formatted page. That way, it is easier to write like on paper and it might be more handsome. There should be a menu in the menu-control that allows the user to switch between modes. That means that there should still be an endless mode and the option to put in a format. For these formats, there should be some presets (DIN A4, letter...) and the possibility to choose one for yourself. This would be really nice.

Things to think about:

• for exporting purpose: the sizeRect method should just return the format
• the pages shouldn't be able to be scrolled out all the way until they're no longer visible (also affects the scrolling behavior)

[jlnalber]

Still TO DOs:

• ~export~
• ~import and export~
• ~change the mode, set formats, preset formats~
• ~(svg preview)~
• ~selector (see #10)~ (there is still a bug, but not because of the format)
• ~(add measuring of distances)~