adds new toggle icons

* for use with compound plugs that have an enable/disable toggle
* contributes to #65
* new icons labeled "toggleOn", "toggleOnHover" "toggleOff" "toggleOffHover"
* no disabled modes - can be added if required
adds disabled toggles

* "toggleOnDisabled"
* "toggleOffDisabled"
Added even-more-simplified mode to StandardNodeUI.

This mode is specified using the new Bare value, and displays the single main column without a scrollbar. This makes it particularly suitable for embedding in other UIs, and in particular the FilterPlugValueWidget is now using it to prevent the triple-scrollbar layout it used to have when being displayed very small with a PathFilter in it.

Fixes #549.
Merge pull request #551 from johnhaddon/scrollLessNodeUI

Added even-more-simplified mode to StandardNodeUI.