Add style.padding in the control stage

[Therenas]

I noticed that, when defining styles in the data stage, we can use the attribute padding to easily define all 4 padding-attributes (top, right, bottom, left) to the same value, at the same time. In the control stage however, this attribute is no longer available, and you have to 'manually' set all 4 padding-attributes one-by-one. I think it would be a nice improvement to have the single padding-attribute still available to us in the control stage, working as a write-only attribute, like width and height already does.

[Boodals]

I have implemented this (I have source access), along with margin. It will be in 0.17.46.