Ability to resize panes

[YaLTeR]

One might want more space for code and less space for preview, or one might want to check how the preview behaves when resized to a small size, without having to resize the entire window.

[sonnyp]

I believe windowed preview better cover these use cases. We already have this if you select Vala in Code but we need an independent control for it.

WDYT?

See also #35

[YaLTeR]

I did switch to a windowed preview later and I guess it works okay for testing sizes. I think resizing the panes still makes sense though, it's a bit cleaner that way when you're not working on a full-blown window type of preview.