Node graph editing UX improvements

[Keavon]

• Drag existing links out of their input ports to be moved elsewhere or dropped (Drag out links to disconnect nodes #894)
• Drag multiple selected nodes around at once
• Copy/pasting and duplicating (Ctrl+D) nodes with their parameters kept (Pasting and duplicating nodes #902)
• Drag a node onto a link to automatically wire it up through its primary input/output (Add drag to connect nodes #901)
• When no nodes are selected, display all nodes upstream of the Output node (Show all nodes with no selection #893)
• Add a hide button and toggle hotkey to mute/disable a node to bypass its primary input/output data flow (Add preview and disable buttons for nodes #905)
  • CtrlH to hide a node, and rename the button "Hide" and "Show" (Node 'Disable' and 'Preview' UX improvements #918)
• Add a preview button to each node to temporarily supplant the connection to the Output node (Add preview and disable buttons for nodes #905)
  • Don't show the Disable button for the Input and Output nodes (Node 'Disable' and 'Preview' UX improvements #918)
  • Alt-click a node to preview it, or disable its preview (Node 'Disable' and 'Preview' UX improvements #918)
  • Explain those keyboard shortcuts in the two buttons' tooltips (Node 'Disable' and 'Preview' UX improvements #918)
• Add a button to re-render (Add a button to refresh the input #926)
• Don't show the blue dashed frame border when the graph is rendered (Don't show the blue outline when bloburl is set #937)

[Keavon]

Reopening till the UX improvements suggested in #905 (comment) are merged.