Redo clone

# WIP!

Based on #295.

This is an attempt to fix the clone failures of #117, but I went overboard when I saw the Clone window and redid it using only "standard" components (aka Open/Save dialogs with accessory views).

TODOs:
- [ ] progress reporting.
- [ ] handle clones started from scripting — this might need the actual cloning to move in PBGitRepository. I'm pretty sure I can't show the progress bar on an empty Main window without triggering failures because there's no repository yet.
- [ ] make the accessory more fool-proof — at the moment if you change your mind after choosing to do a local clone, you have no way to get back at the remote URL field.