Issue related to undo move backdrop nodes

[itscassie]

When using backdrop nodes
Under the scenario that I got several other nodes wrapped in a backdrop node,
when I move all the nodes wrap in backdrop node, and then undo the "move nodes" action (using the context menu)

sometimes I was unable to move all the node back to the original position,
the backdrop node itself move back, but the nodes it wrapped remains at the same position

The frequency of encountering this bug is around 1/2, not sure what cause this happens.

[itscassie]

I have found that there are different use cases that result in different outcomes:

1. click and hold the LMB to select the backdrop node, then drag without releasing the LMB:
   only backdrop node will be undo
2. click on LMB to select the backdrop node, release LMB, click LMB again and drag:
   all nodes wrap in backdrop node will be undo

More specifically, the scenario differs based on the "select" signal when moving nodes:

1. If some nodes were auto-selected by selecting the backdrop node and then moving => unable to undo/redo
2. If all nodes were selected before moving => able to undo/redo

There are two circumstances under which mousePressEvent will be triggered:
a) In NodeViewer
b) In BackdropNodeItem

a) when mousePressEvent is triggered in NodeViewer:
it will update its _node_positions property by finding selected nodes and recording their view and current xy position.
b) when mousePressEvent is triggered in NodeMoveCmd:
it will find _node_positions and see what nodes are recorded in it.

When selecting wrapped nodes by clicking a backdrop node, the select event is in the mousePressEvent of BackdropNodeItem, and the wrapped selected nodes are not counted in selected_nodes() of NodeViewer.

[jchanvfx]

Hi @itscassie,
I've just made a new release v0.5.8 with some tweaks to the backdrop selection logic.
https://github.com/jchanvfx/NodeGraphQt/releases/tag/v0.5.8