Clear selected categories before applying those from command-line

This fixes #22, which complains that categories selected by /include or /exclude on the command-line are merged in with categories that were selected in a previous run and saved in the visual state.

We don't have any tests for this and I didn't add any but observed the affect of running the gui repeatedly with /run and both /include and /exclude options. The categories selected on the command-line replace any that were selected in a previous run as requested.

This entire feature will need work in NUnit 3.0. Selected categories impact what tests are run and are therefore more than just a "visual state." As such we may want to save them elsewhere or possibly not save them at all. The work here should do it for 2.6.4, however.