TST Folder Expand Collapse per dsl101

TST 'Folders' (e.g. URLs of the form moz-extension://[TST-Ext-ID]/resources/group-tab.html) are toggled (expand / collapse) when clicked, instead of being activated. Regular tabs which have children will do the same when the CTRL key is pressed in addition to the click.

Note when hibernating a tab, TST tries to focus & activate the parent. This is disabled when using this add-on, and the next previous active tab is focussed instead. This prevents 'folder' tabs from becoming activated when hibernating their children.