Miscellaneous UI tweaks.

- Using a simplified form of StandardNodeUI when embedding the Filter ui inside FilterPlugValueWidget.
- Placing filter inside its own tab for FilteredSceneProcessors.
- Placing transform inside its own tab for ObjectSource nodes.
- Placing Shader name widget with Reload button in the header of the NodeUI.
- Allowing any callable that yields a NodeUI instance to be passed to NodeUI.registerNodeUI, rather than just NodeUI subclasses.
- Removing user tab from preferences ui.
- Improving ordering of StandardNodeUI tabs - Settings tab now always comes first.