groupbox/tasklist: Better X/Y margin/padding settings

Paddings can now be specified as X and Y separately, and margins can be specified as just "margin" too.

Specifying `(margin|padding)_(x|y)` overrides `(margin|padding)`.

(Doing this because personally I like to set padding_x to be wider than padding_y. It creates a nice effect with the background color)

Example:

![](http://dump.dequis.org/3_yu5.png)

(Note the wider horizontal padding in the selected ones)