Enhancement: Add shortcuts to duplicate, rename, delete or group selected prims in prim hierarchy

[BigRoy]

Enhancement

Add shortcuts to duplicate, rename, delete or group selected prims in prim hierarchy Also enables multiselection in the prim hierarchy.

Implements #23

Shortcuts

• [x] CTRL+G: group selected nodes into an Xform named group
• [x] CTRL+D duplicate selected nodes
• [x] Delete: delete selected nodes
• [x] Enter: edit name in view on current index (like F2 shortcut)

Skipped the following - those are better left for individual PRs:

• [ ] Tab: create nodes quickly from a type search popup menu (like e.g. creating nodes in Houdini, Nuke or Fusion)
  • This feature is sligthtly more involved because it needs its own "quick search" widget so might be best left for another PR.
• [ ] Shift+click expand/collapse: Expand or collapse all for the selected expand/collapse action (like maya)
• [ ] F: frame the current selected node? Or do we like the default tree view behavior where typing brings you to the closest name.