Improve placement of floating widgets

[aarranz]

This PR improves the FreeLayout class to add more options when placing widgets. Those additions consists on:

• Supporting different placement origins. Currently floating widgets are placed using the top left square as reference. This PR allows to use other reference points: bottomleft, topright, bottomright, topcenter and bottomcenter. Once changed this option, widget coordinates (x, y) will be taken relative to the selected square.
• Supporting selecting between proportional and absolute sizes and coordinates. Currently floating widgets are using proportional sizes and coordinates, meaning that when the dashboard changes of size, those parameters are adapted to the new size. This way you can select the desired behaviour.

[coveralls]

Coverage increased (+0.7%) to 82.699% when pulling 6cc5a42 on aarranz:feature/rel-freelayout into 3ca4e4a on Wirecloud:develop.