Added NodeToolbar and StandardNodeToolbar classes.

These are refactored out from the private Viewer toolbar class. Currently they are still only used to present plugs for the Views, but this is a step towards having toolbars for any of the standard nodes.
Removed unnecessary import from GafferUI.__init__.py
Fixed StandardNodeToolbar for dynamic addition and removal of plugs.

This is using the same method as is used in StandardNodeUI and CompoundPlugValueWidget. The method itself has flaws (although they're much more likely to be an issue for CompoundPlugValueWidget than for the toolbars), so I've added some more notes as to how we might be able to address them.

Also added workaround for labels having an unwanted width, and a todo for getting it fixed.