resolve #85 and added open file dialog functionality to qml frontend

Added open_folder command + test and also implemented it in the qml frontend. Created an additional folder for dialogs to seperate the different qml elements a bit(but can undo it, if this way is not wanted). The opened file doesn't get syntax highlighting so far, because i didn't found anything, except for hardcoding it myself. So I let it be, because I think there will come a dynamic syntax checker later, which will set the syntax automatically :)
Yup, it's only been possible for [about a month](https://github.com/limetext/lime/commit/f65292060df4376261610b1f2e7cb1f202074fa7#diff-81a5d95ff306cc5cb1a4659b90a8e264R99) though.