Add data panel

[slimbuck]

This PR:

• adds a new collapsable data panel below the 3d view
  • the view displays histogram for currently selected splat data
  • user can choose to plot one of the following properties:
    • X, Y, Z, Volume, Surface Area, Scale X, Scale Y, Scale Z, Red, Green, Blue, Opacity, Hue, Saturation, Value
  • click and drag on histogram to modify selection (using shift to add and ctrl to remove from selection)
  • hover mouse over histogram to see bucket values and counts.
• removed 'Select by opacity' and 'Select by Size' from control panel since histogram select is much more powerful
• bumped package version

[willeastcott]

Can't wait to see what people think of this update!