Improve support of submodules in SourceTree

1. If different submodules are specified for the different branches of the same project and some of them are cloned into same local folders, it's almost impossible to switch between branches.
SourceTree reports that "cannot remove folder because it's not empty", looses track of submodule, shows infinite number of dialogs that "repository is missing" and so on.
Meanwhile, Tower just switches branches and get all submodules for the branch without that hysteria, so it's possible to implement.
2. It would be great to indicate when you in detached head, because it's quite easy to loose your changes.

Guys, I don't quite sure whether it is Bug or New Feature, so please set proper issue type. Also, I marked it as Critical as switching between branches which have different set of submodules is an everyday operation for us, and all my team members are quite tired with that "submodules nightmare".