Exposed "Restart Presentation Compiler" action

Users can now restart the presentation compiler at will.  This can be handy
when false errors are reported in a compilation unit, and you want to reset a
Presentation Compiler without performing a full clean/build.

To manually restart a Presentation Compiler, a user has to right click on the
project's folder (in the Package Explorer), and select ``Scala > Restart
Presentation Compiler``.

In order to place the functionality inside the Scala menu (as described above),
I was forced to use an `action` (and not a `command`). I did try to use a
`command`, but I couldn't find the incantantion for placing the "Restart
Presentation Compiler" inside the Scala menu when using a `menuContribution`.

Finally, there is also a shortcut to trigger the implemented functionality,
which is ``SHIFT+R C`` (for Restart Compiler). As you can see in the code
itself, creating a key binding for an `action` requires you to create a
`command`. Not ideal, but I couldn't find any other way of achieving this.

Fixes #1000555