Add origin to the Transform node (and visualize it with an overlay) #1530

Design and implement the new pivot system containing a pivot stored in layer space (?) instead of normalised bounding box (so a pivot of (0.5, 0.5) would no longer be in the middle of the bounding box as it is currently). Note that this will break things such as rotating around the pivot, transformations applied by tools, etc. Careful thought should be applied as to how exactly the pivot should be stored.

"Pivot" concept is split into "reference point" and "pivot"

• Reference point: after the graph has rendered, this is the point from which we measure X and Y location
• Pivot: an Option<> which starts out as None, it must be set (either by a Set Pivot node or a widget next to anything, like Scale or Rotation, which requires it)

Also we likely don't use the nine-square widget for pivot to ensure people think of it only as the reference point concept

So the Transform node changes like so:

• "Pivot" becomes "Reference Point"
• Rotation and Scale get a "Pick Pivot" widget which lets you place a pivot in the artwork, it's required if the pivot is None
• Also: add skew? And non-affine?

Goals: don't have shift caused by upstream changes, thus we can't use the bounding box as a pivot reference, instead we have to explicitly have the user set any desired pivots

As part of combining issue #534 into here, here's the description that was part of it:

• Folders have transforms?
• Showing skew in properties panel
• Non-affine perspective transforms (dragging each corner of the bounding box to an arbitrary place, except concave)
• Inconsistent behaviour between lines / polylines and other shapes (render_index in simple_shape)
• Allowing outline width to be scaled / not as desired
• How to display layer & folder(if applicable) transforms in properties panel (i.e. do we show layer transform or bounding box, and how is it transformed etc.)
• Clear reference for contributes / artists