add methods to DocumentLayout

[maraky]

Hi!
I propose to add methods to DocumentLayout, return the size of the slide in px, as the height, the width of the object and its displacement is set in px. This will avoid unnecessary conversions and make the code more readable.
Thanks, Alexander

[Progi1984]

@maraky I just added units management in DocumentLayout.

You can get and set width and height in differents units (pixels, emu, cm, mm, pt and inches).

This is available in the develop branch.