Preview panel 3

Last few fixes for the Preview panel, rendering settings, main viewer...
To test:
 - Right panel General tab: button renamed "Launch full viewer" to match Preview tab.
 - Right-click menu on Tree for rendering settings: "Set Owner's and Save" moved to last (same as Insight).
 - Zoom buttons on the image viewer (and preview) won't zoom below 0%.
 - Z/T active sliders are white instead of red (any suggestions of a better colour?)
 - Full viewer has "Min/max", "Full Range" and "Imported" buttons, should behave same as in Preview panel.