Add LuaStyle.extra_[margin/padding]_when_activated

[Therenas]

Currently, when changing styles during runtime, we have nice convenient attributes on LuaStyle where we can change multiple of the directions (top, right, bottom, left) in one line, simply using .padding and .margin (and even .cell_padding!).

It would be swell if we could have the same kind of attribute to set 'extra_margin_when_activated' and 'extra_padding_when_activated' in the same way.

[Bilka]

Added LuaStyle::extra_margin_when_activated and extra_padding_when_activated write, which also accept arrays of values.