#65 Add minsize, proportion to gbsizeritem

[Christoph Theis]

According to http://docs.wxwidgets.org/trunk/overview_xrcformat.html#overview_xrcformat_sizers minsize is a property of all sizeritems.

growablerows and -cols should support proportion (e.g. 1:1), this means those entries are no longer pair of integers.

sizeritems may have the "option" value set to "proportion", though documentation is not clear here. The code does use this value for all wxSizerItem