Trigger select's onChange via React
Fix shortcuts list

(This most probably just a temporary solution!)

There was a problem the way the shortcuts list was being built after the latest changes.

First of all it was build everytime the render() method was called, which wasn't really optimal.
Secondly, the code kept adding shortcuts in shortcuts.list for the 'Answer' and 'Hint' dropdowns (so you ended up with 'C', 'CA', and 'CAT' as shortcuts for 'Categorizer').

The way I fixed it is to use the shortcuts list as an instance variable of each WidgetSelect so that they don't conflict with one another.

This is obviously not optimal as the list should be only one, shared between the element instances. I tried to build the shortcut list as a 'global' variable but Widgets.getPublicWidgets() return only an empty array up at the beginning, so for now I wrote this solution as a baseline for future improvements.