Draft: Added support for editing arc segments in the node editor

Hi, I'm a new contributor. I've added support to the path editor for editing elliptical arc segments, which were previously converted to beziers on the fly when editing.

The most notable differences are;

• Arc segments now get a pair of handles at right angles to each other, to manipulate the two radii of the ellipse and the rotation of the ellipse.
• Arc segments are no longer automatically converted to beziers upon editing a path
• Arc segments can be converted to beziers using the "make segments smooth" button
• Dragging the line of an arc segment will not manipulate the segment like it will if the segment is a line or a bezier (and therefore not all segments can be dragged, whereas they could before) (maybe a different behaviour is desired here)
• The arc flags (the large flag and sweep flag) can be set using new buttons in the node toolbar

Current state: