[Epic] GraphNode QML GUI

[RobBuchananCompPhys]

Since I've started receiving feedback on the PR #2566 , I figured I'd start collating the various bugs/hiccups/further-improvements into an issue, in order to keep track of everything. The overarching objective is to put these up for discussion amongst the team so these categories are currently provisional. I'm including things that are considered good, just to maintain a general picture of what's been added - we may well want to revisit these at some point.

Things we like

• Node headers "grow" when hovered
• Node parameter description banners/pop-ups

Things that could be improved

• Edges render even when the endpoint types are incompatible
• Rendered edges disappear when a partner node is destroyed, but do not respond when the connection is manually removed (in fact there is not currently a way to do that, either). Perhaps double clicking on the input/output to destroy the connection?
• Resurrect the right-click Graph menu for adding Nodes -> what do we think of making Node category part of the NodeRegistry? (I prefer this to the information being stored in the QML) (refactor: Categorical NodeRegistry and updated QML Graph menu #2582)
• Nodes added to the graph via "Find Node" wizard always spawn in the same location - should ideally select a free location in the graph
• Parameter description pop-ups could instead be tooltips at mouse location
• Root Dissolve graph I/O nodes should be invisible by default
• Cubic edges (fix: Return cubic shape to edges #2578 )
• Feedback to user on graph run (i.e. Right-click on node -> Run) - how much/where should information be relayed?
• Add node should have spinboxes next to the X/Y inputs showing the current / edge-derived value
• Would be nice in the future if we could link up the Number option/output, rather than having them separated
• "Find Node" wizard should be fully populated by default
• Serialisable Parameters don't appear to display on nodes - should they?
• Subgraph/IterableGraph proxy I/O doesn't show up yet. Ideally, you should be able to make arbitrary connections to the proxy inputs, for instance