New dialogs

Hi there :)
I've been working on some new dialogs for a while and I think it's ready to merge now.
Some notes:
* The dialogs created when clicking the titlebar close button or hitting Alt + F4 are not modal, I haven't figured out how to do that yet (they are modal when launched from QML).
* For some reason the path of the initial folder for the FileViewerDialog's will not be displayed when launched from QML, but will when created as a component from `main.py`.
* Changing folder in a FileViewerDialog causes masses of errors, I think because `FolderListModel.get()` returns a `variant` (line 133), which technically may not have a `.toLowerCase()`. Would be nice to suppress these if no solution can be found.

ToDo: 
I'm also a little unsure of the way the code works with `EventFilter()` in `main.py`. Dunno if it's the cleanest way to create the dialogs.

This fixes #72.