[folding] Folding icons should not operate on mouse-down

[weinand]

On testing #2994

I always hit the folding icons by accident and that collapses the code unexpectedly.
To improve this I suggest to do the following:

• make the folding icons only operate on mouse up.
• detect the difference between a drag-operation (e.g. to select a range of text) and a fold/unfold. Only if the mouse-up location is very close to the mouse-down location, perform a fold/unfold. Otherwise assume a drag, that is start a line select. While dragging inside the folding icon, the icon should probably give 'pressed' feedback like a button.