app: allow file:// URLs

Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola.desktop: associate with inode/directory

Add a "Bookmarks" action that does the traditional
`git-cola --prompt` startup dialog.

Change the default Exec to `git-cola --repo %u` and add the
inode/directory MimeType.  The mime type association allows us
to open directories with git-cola from a file browser.

Suggested-by: Muhammad Bashir Al-Noimi <mbnoimi@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>