BUG: Duplicate class definition in TestSeedWidget2

The class vtkSeedCallback was defined in both TestSeedWidget and TestSeedWidget2
causing memory issues and dashboard failures.
Modified the tests to use the new vtkTesting::InteractorEventLoop API.
Added custom interaction to TestSeedWidget2.

Change-Id: I93eed595ca950407dc5e114d030fb93008ceb7c3
Merge topic 'fix_TestSeedWidget_failure' into master

7d5b2480 BUG: Duplicate class definition in TestSeedWidget2