PlugLayout : Added summary tooltips to tabs.
PlugLayout : Added summary tooltips to tabs.

These are identical in nature to the summaries shown in the headers of collapsible sections, and are generated from the same metadata.

Fixes #332. This ticket asked for description metadata to be displayed, but since tabs are layout items rather than plugs, only the section summary actually makes sense. I think the ticket dates back to a time when we didn't have section summaries.

boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,hradec/gaffer,boberfly/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,chippey/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,chippey/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,chippey/gaffer,lucienfostier/gaffer,chippey/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,hradec/gaffer,hradec/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,boberfly/gaffer,GafferHQ/gaffer,boberfly/gaffer,hradec/gaffer