Make it easier to select and move Lines

[raimohanska]

Could start by

• Somehow highlighting the line ends / center point when hovering over
• Allow moving the whole line/connector when dragging from the center, in case it's a straight one. This could be done by adding a controlPoint in the center (but keeping it invisible except when hovering) and having special handling for straight lines (move whole line when dragging). A slight remodeling is needed to be able to represent a line that has a control point, but is not a curve. If either end is attached to an item, that end of course cannot move.
• Possibly adding more "invisible control points" for even easier grabbing