Add preference settings for selection size

[jdujava]

Problem:
Selection can be sometimes unnecessarily big (mainly when working zoomed in). It would be suitable to have a preference setting for selection padding, such that one can adjust it according to their workflow.

Two options added:

• SelectPaddingMult (to change selection padding)
• ButtonSizeMult (to change size of selection buttons)

This is still WIP (need to add some tooltips and other small tweaks), but overall it seems to work pretty well.
I will gladly hear your thoughts on this.

[bhennion]

I'm not sure this is the right solution to the issue. I think the problem comes from the selection box padding not adapting to the zoom (i.e. there should be a padding of fixed screen dimension, not of fixed page coordinate dimension).
In any case, the whole selection business needs a complete overall -- split model-view-controller + optimize out a lot of useless computations (I have a draft in the making somewhere).