Disable "detach-tab" after closing others

Other signals are received too early when closing tabs.
ViewContainer::viewRemoved is received after the tab count has been
updated.

Protect against the "detach-tab" action when only one left.

Fixes the following behavior:
 - Create two tabs
 - Close one tab
 - Detach the remaining tab through menu or shortcut
 - The original window is blank - no terminal display. Adding a tab
   to it crashes