User settings page

While working on the email notification module, which adds an own local tab below /messages just for a single enable/disable checkbox, I thought it might be nice to have a generic settings page for the user. The problematic thing is that there are no settings provided by privatemsg.module itself, so modules can't simply use hook_form_alter to extend a already existing form.

My idea was to add a privatemsg_user_settings hook, which allows to return form fragments. The settings page is then only displayed if there are such form fragments and the user is allowed to access them.

Attached is a first patch, I'm setting it to CNW to get some feedback, but it's not ready to be commited, it contains dummy code to test the hook and no real implementation yet.