Backport (7.8) - Show more information about the chosen variant and auto-select its tab in the component properties window

Umbrella issue for various improvements in the template composer related to variants:

• In the combobox, the chosen variant should be displayed as "<name>", or "<name>, the <group>" when a group is available for that variant. It's tooltip should also contain its description, when available.

• The variant selection combo should be usable in both view and edit mode.

• The selected variant should be 'remembered' when switching back to view mode, and when switching channels.

• When selecting a component, the tab for the currently rendered variant should be auto-selected.