Added dropdown of available transition actions in dialog.

Today I'm attending Process Management training.
I found it very unpleasant to have to type in

  Kernel::System::ProcessManagement::TransitionAction::TicketStateSet

every time I want to add a transition action. It's a long string and it's error prone.
I added a selection that shows a list of available transition actions.

Of course you still need to know what key-value pairs are needed as arguments for the
transition action, but at least entering a new transition action now is a little more
convenient.

Old screen, no dropdown:
![dropdown](https://f.cloud.github.com/assets/659504/674686/245da0d6-d8e0-11e2-872a-b075b6637d22.png)

Now with dropdown:
![transitionactionnew](https://f.cloud.github.com/assets/659504/674681/09783ba0-d8e0-11e2-94c8-5acbb40ed324.png)