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 :)
This function can be completely removed as the functionality will be [automagically done for you](https://github.com/limetext/lime/blob/763d2d2472dc10599bca3a01f1cab03b530bd394/backend/commandhandler.go#L48).