Added useNameAsPlugName argument to CompoundDataPlug::addMembers().

Also added python bindings for CompoundDataPlug::fillCompoundData() and CompoundDataPlug::fillCompoundObject().
Gave Displays node parameter plugs more useful names.
Added dropdown menu for Displays node quantize parameters.
Improved Displays node UI.

- Displayed only the label in the header section.
- Added file browser for name widget.
- Added a widget for editing the label in the collapsible section.
- Greyed out widgets when the display is turned off.
- Added delete button, visible when hovering over a particular display.
- Stopped deriving from CompoundPlugValueWidget and overriding _footer() and used PlugLayout instead - one less thing to deal with in the ongoing refactoring of plug UIs.

Fixes #15.
Merge pull request #823 from johnhaddon/displays

Displays UI