Allow to move files and directories via Drag & Drop

This implements drag & drop for the tree view, as wanted in #55. It supports dragging files and directories.

It needs some interface love like an indicator to see where you move the file to, some delay for expanding directories and a better view when you actually drag directories.

This is how it works (don't know why the GIF is so choppy):
![tree_view](https://f.cloud.github.com/assets/82050/2371943/b68d4a4e-a831-11e3-93b8-e5c4f9f3244f.gif)

I have code ready to allow dragging external files into the tree to add them to the directory, but don't know what the behaviour there should be. Moving the files/directory or copying them? Or maybe a setting? I just need your input (/cc @SebastianSzturo) and I'll implement that.

Feedback is very welcome, I'll give it some interface love tomorrow.