[vtkusers] Handling "holding down" a key

[David Doria]

>> Hi David,
>>
>> Clicking the 'x' key to change the state is preferable.  You just need
>> to add a visual cue to inform the user of the current state, e.g. you
>> can change the shape of the mouse cursor or draw a horizontal arrow
>> somewhere.
>>
>>  David

@David G.

I made branch here:

repo: git at github.com:daviddoria/daviddoria-vtk.git
branch: VTK-TrackballActor

Would you mind taking a look?

(These two commits:
http://github.com/daviddoria/daviddoria-vtk/commit/687ed4819ae61e169ca016e3442e9f778b0ef5d9
and http://github.com/daviddoria/daviddoria-vtk/commit/1671de6acf35a3d1cb4cd44f1fdb481b50e17e3e)

As for "change the shape of the mouse cursor or draw a horizontal
arrow somewhere." - it may be unnecessary overhead to force a text
actor or some other orientation marker into the scene. It really isn't
that hard to keep track of mentally :)

@Jim - just as above, I didn't want to force an orientationWidget to
be present to allow for generic use of this.

Thanks,

David