too easy to lose settings changes in rez-gui

If you follow these steps:
- open rez-gui;
- click 'settings' tab, make some change;
- click back to 'context' tab and resolve;

..your settings changes are silently lost (you needed to have hit 'Apply').

Fix: When on settings tab, grey out other tabs until user either hits 'Apply' or 'Discard Changes'.