Use GTK# for Linux

[Miss-Inputs]

I'm beginning to realise I really just gotta do this. It'll solve all the things. It'll probably solve #20. It'll make it usable on high-DPI screens.

Also by having multiple frontends by way of multiple build targets, we can probably have a CLI frontend too, which will solve #6 and #10. This should help with #9 as well.

Since the only reason we're hanging on to WinForms is portability, this also gives us the option to use WPF for Windows. That doesn't have to happen right away, as technically WinForms does work under normal Windows, but it becomes an option once we do the thing.

I'm not sure how this all works on Mac OS X, and I may have to enlist the help of someone else to ftest stuff, since I don't exactly have the hardware to run modern Mac OS X. Not sure if there's other platforms Mono runs on either. Until I can find out though, I'd better keep WinForms around.

Also I've just wanted to try GTK#.