Add a scrollbar in flowable-task to be able to select (and start) any process

Hello,
If we have a lot of processes imported, we can't see all of them when we click on `Start a process` button in `flowable-task`.
Currently, the only way to select a process at the end of the _list_ (it isn't a list in fact) is to decrease the size of the font... :)
It would be good to have also a scrollbar for the list of processes to start (like the list of processes).



Thanks for your work
Regards
William