Improvements: Graph Editor

[natowi]

For graph redesign:

• when hovering over a connector also highlight the start and endpoint on the nodes (and/or when hovering over the node option - at the moment the text is only highlighted blue)

• click on connector: select connector; also highlight the start and endpoint on the nodes and make the connector detachable (to attach to other node) instead of needing to remove the connection first. Enable Del button to remove a selected connector

• when creating a new connection, pan Graph Editor view when cursor is near the border. Otherwise you have to zoom out first. (similat to Infinite drag #993)

• centre node option text for options with input and output. Keep Input only node options left aligned and Output only options right aligned.
  This way it is easier to find the corresponding text to an output node.
  (maybe also centre node name (StructureFromMotion) instead of left aligned)

• fix auto layout depth mode minimum or remove it, as it is not helpful at the moment

In the future:

• colourize nodes based on categories used in More intuitive add node menu #248
  (could also be optional)
  Sparse Reconstruction, Dense Reconstruction, Utils, Mesh post-processing
  Similar to Blender:
  

• Insert: default pipeline / Draft Meshing pipeline

• add supported node: add "add supported node" to context menu when clicking on a node connector. Show the add node dialogue with node compatibility check enabled:
  Distinct between supported input nodes and supported output nodes.

Implemented:

• collapse and expand nodes to simplify the node graph and expand them if needed, auto connect supported inputs/outputs when collapsed

• Improve node connector routing. Nodes should not overlay connectors
  [transparent node]

• 5 - right click "Delete nodes from here" Implemented

• 9 - bidirectional node connection (allow connecting the output to the input of supported nodes (7))

• 10 when passing data through a node (A->B->C) do not (by default) connect C to input of B, connect to output of A or provide better connector routing (1) or create a pass-through output for B. (implement new node connector behaviour)

Many features implemented in #612

• Multi select nodes to move them: Ctrl+mouse (to select multiple nodes) and mouse area [] select # 1227

Ctrl+mouse-select

• mouse area select [ui] add support for selecting multiple nodes at once #1227

[natowi]

Node Graph redesign: #612