Fix leaving the card keys view with Shift+Tab

If one of the action buttons (which are item delegates) has keyboard focus
and one presses Shift+Tab to leave the card keys view then Qt gives focus
to the tree widget. The tree widget gives focus to the current item
delegate and we are back where we started.

Break this focus loop by removing the tree widget from the tab chain
while one of the action buttons has focus.

(cherry picked from commit 5041278d8200f39b36986d82005261c07ea7b3ad)

• GnuPG-bug-id: T8051