Add applet from widget explorer on single click instead of double click

Merged Arjen Hiemstra requested to merge work/ahiemstra/w-e-singleclick into master

It is unclear that double click is required here, especially since there is no selection state. So instead, use single click to add an applet. This also makes things easier when using touch, as double tap is quite uncommon and the long press to start a drag is also not very intuitive.

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