Allow rearranging items in lists by dragging them

[kduske]

Currently, list items such as layers in the layer list, or compilation tasks, are rearranged by clicking the triangle buttons below the list. It would be nice if they could be rearranged by dragging.

Note thought that these items have active controls in them (buttons, text boxes, ...), so we can't easily allow dragging them from any point. The items should have dedicated drag handles. Drag handles are often represented by three parallel horizontal lines.