Refactor panel spacer flexible auto-sizing code and implement multiple areas
The old code only supported centering one widget. Now, by adding multiple spacers, you can make sure that widgets will be aligned to 25%-50%-75% of the panel (if you have 4 spacers), or 33%-66% (if you have 3 spacers), and so on.
Also, I remove a lot of code; I think I preserved all existing functionalities?
Edited by Niccolò Venerandi