z-Secret: tabs.getSelected->tabs.query, refactor codes, closing tab
z-Secret: tabs.getSelected->tabs.query, refactor codes, closing tab

1. As tabs.getSelected() is deprecated, it needs to be changed to
tabs.query().
2. To abstract this feature and to obey convention, some codes need
to be refactored.
3. Now, z-Secret can closes current tab before opening a new
secret tab.

Signed-off-by: drzix <75c00122d1417de46f33afdfc0ba3516a681310e@gmail.com>

drzix/z-Chrome,drzix/z-Chrome,se0kjun/z-Chrome,se0kjun/z-Chrome