Support for percent based font sizes.

There is currently no way to specify text to simply be larger/smaller which is very problematic when developing for multiple platforms (which is why so many people use Qt!).

For example if you set the pt size of the default font on Windows to make text bigger that can actually make the text smaller on Mac because the default pt size is so different! To solve this problem it should be possible to write something like "font-size: 150%" instead of being forced to use pt values.