Dynamically update category items

Since projects can be opened and closed all the time, and the display of
our category items depends on the opened project paths, we do the
following:

• listen to IProjectController::project{Opened,Closed()
• update the internal cache of project paths when that happens
• visit all the category items and update their texts (The full path, which doesn't change, is conveniently stored in the tooltips)
• Also use QDir::homePath()

This makes display of the categories work without hardcoded pathes.