Make the icon view react to single click instead of double click

Qreator uses a Gtk.IconView widget to select different types of elements. The way the IconView works is the following:

* Do one click to select the item
* Do an additional double-click to activate the item

I've always found it confusing that an additional double click is needed, when most button-like widgets in the desktop just require a single click. Now I've just had feedback from a couple of users that find it confusing (someone thought it was a bug that clicking on the button "did nothing"), so I've decided to look into how to change that behaviour.

Ideally, I'd like the IconView to behave as follows:

* Hover to select an item (optional)
* Single-click activates the item

See the related AskUbuntu question at: http://askubuntu.com/questions/144311/how-to-make-a-gtk-iconview-react-to-single-click-instead-of-double-click/144313