Font size property in stk::label

If noone minds, i'll add a font size property to stk::label, imo this 
seems like the clearest way for me to have different label sizes in the 
gui, to leave control over the default font size to the theme i'am 
thinking about using boost::optional, so that if no value is specified 
the theme may decide up-on the default size.
I would greatly appreciate some feedback about such property's because i 
might run into the need for these more often in my applikation, we 
should maybe discuss some standard policy about display relevant widget 
properties.

Marc