[tab-group feature suggestion] Add a 'current tab' notion in tab groups

I like the tab group feature to organize tabs, but clicking a tab group tab has a completely different user experience as clicking a non-grouped tab.

By adding a current tab in a tab group, the user experience can be more unified: clicking a tab activates it (brings it to the foreground), clicking a tab group tab activates the current tab in the tab group. This allows for quickly swithing between the 'documentation' for the tab you're working with and the 'project' for the tab you're working with, for instance.

The current functionality of 'expanding' the tab group can be moved to the right-click context menu.