OCIO contexts round 2

[johnhaddon]

This is a squashing of @lucienfostier's work from #1984, plus a UI tweak I've added so that the user can only add string context plugs.

Lucien, everything here should be as you left it, except the following :

• Fixed bug whereby m_hasContextPlug wasn't initialised to false where necessary
• Made the ocioContext() method private and non-virtual
• Added the custom UI for adding strings only
• Cleaned up a bit of trailing whitespace

Could you give it a quick check over and let me know if all is as you intended?

Cheers...
John

[lucienfostier]

Thanks john,

loops good to me!