Add separator menu item which will appear below the Clear Items button
Add toggle switch for entering clear mode
Clear toggle now works as intended:

- Clear mode allows multiple entries to be selected
- Exiting clear mode removes the selected entries
Refactor to use states and transitions; also fixes #5
Fix removing of selected item and removing of all items; add an empty state;
hide clear mode toggle when list emptied (which stops the menu from popping up
altogether); select last item in list (if there is one) when currently selected
item gets removed - fixes #3 #4
Merge branch 'master' into feature/clear_items

Conflicts:
	extension.js
Remove unnecessary timeout halt/restart
Fix bugs with zebra striping falling out of sync when items are added/removed -
fixes #7
Merge branch 'feature/clear_items'