Reorganized clone dialog

Here it comes! A polished version of our clone dialog. Not everything finished yet, but works in general.

The dialog is expandable to show more options, but in its initial state it shows the very simplified version. 

This is crucial for the following reasons:
* Especially new users to Git will appreciate a small user interface without the screen flooded with options (even if they are all preset with the correct values).
* The workflow must be reasonably fast. Thus, it should be editable just by the keyboard at hand.

For everything else, there's the options panel. When expanding the options, we have three "modes" available (currently titled with "Repository Type"):

* Normal: Creates a normal Git clone of an existing repo, followed by a checkout to the working directory.
* Bare: Creates a bare clone. In this mode, submodule options are not available.
* Mirror: :construction: (not implemented yet) This is a mirrored bare clone of an existing repository.

- [ ] By default, append the repository name to the destination path. For a bare clone, it should end with `.git`
- [ ] States are not completing correctly, which is actually still a :bug:.