Added font rendering with freetype-rs and implemented label widget.

The example has also been updated with the new fancy Label business!

Unfortunately the example now crashes upon close due to a known "double-free"ing issue in freetype-rs, though I think @Coeuvre might be working on a fix for this?

Also, performance now hugely fluctuates when drawing labels or widgets with labels. I've tried to follow the usage example within the `font_with_piston` freetype-rs project as close as I can, but I haven't yet been able to fix it. I'm unsure if this is an issue with my implementation or if the problem stems from within freetype-rs itself. On some occasions the example runs blazingly fast without issues, however most of the time the example will start fine but progressively slow down to an unusable state. If someone gets a chance to run the example and has some ideas, that'd be great! Otherwise i'll revisit tomorrow and have another stab :-)

Once the bugs are sorted with the font rendering I'll have a go at a DropDownList and a NumberDialer... and maybe even a TextBox. Quite excited about making progress on this :)