[crop] Add disable_window_dragging option, enable by default

[po5]

This lets users hold and drag to select a crop area.
The window-dragging property is temporarily set to no, can be disabled with a script opt.
After dragging, releasing left mouse will submit the crop.
It is still possible to do a simple left click to set each crop corner individually.

[occivink]

Sounds good, but I would like it to check the state of the property on down event, and only re-enable it if it was actually enabled.

[po5]

True, will do that.