Justacec/remove projects from menu

Previously I had looked for a way to clear out the list of recently used projects from the projects popup menu.  I was instructed that the only way I could do so was to remove my settings directory and start fresh.  I decided that this morning I would see if I could add a menu option to the projects menu to clear this out.  After some investigation I saw that most of the code was already in place.  I have added in the necessary code to do the minimum thing which is to simply clear the MRU list for the projects.

As indicated earlier, this is the minimum thing that can be done.  I think that there a several possible improvements:

 1. Allow the user to selectively remove individual popup items from a dialog window that would be dedicated to the management of the MRU instance for projects. (Or have action buttons on each project MRU item, say a little "x" symbol, that the user could click which would remove the need for a popup.)  
 2. Do a sanity check and automatically remove the MRU item if the corresponding project file does not exist.
 3. A combination of 1 and 2.
 4. Remove the menu separator and the menu item to clear the contents if there are no current items in the MRU list.
It turns out that this handler is actually not required (since the command is already handled by the ProjectMRUList class) so let's remove it.