Clamp the size of the TreeView to a minimum of 0

[liquidev]

Fixes #44. I'm not sure if there's a more clean way of doing this, I scoured the RootView's code but couldn't find how the sizes of DocViews are exactly clamped, so I did this. Hopefully it's not too horrible!

[budRich]

I tested this and it works great. I also tried resizing it to 0 just to see that i was then able to grab the border, and that worked as well. @franko in contrary to myself actually knows Lua and can have a look this before its merged. But I think the solution looks really elegant, probably useful for other things as well.

[franko]

Good, thank you but I already fixed in a less ad-hoc way. I was in a better position than you for that because I know the lite resize system and I modified it recently for the 1.16 release.

My fix also clamp the treeview resize on the other side!