@Noughty - the reason why it is not a double click is because at the moment there are two *distinct* possible things you can setup for a shelf, which go to different dialogs. People who use the "Add to shelf" functionality setup those actions, and people who use the "sync" feature set those actions up. So that means a double click feature would need the option to choose which set of actions it is going to launch. Which all sounded a bit complicated which is why it doesn't currently do that.
Another possibility would be to just have a single dialog for a shelf that combines the two with a couple of tabs on it, one for the sync actions and one for the add shelf actions. However that has the potential to make things more complicated. For instance the user would need to be aware if they have made changes on the other tab. And for the case of editing multiple shelves at once, you have the potential to do some unwanted things, i.e. if you had a sync action setup for one shelf, selected multiple shelves to setup add actions and then end up with that sync action on all of them. It just gets real ugly.
You know there is a right-click menu as well right? So rather than double click or single click and move to the button, just right click on the shelf and select the menu item you want.
Since you can do multiple shelves at once, I wouldn't have thought there was a massive need to support a configurable double click as I wouldn't expect people to be individually editing actions for lots of shelves? However if others are also keen for it I can take a look...