contacts-main-window: deleting a contact now removes their name from the headerbar

As described in #128, deleting a contact causes that contact's name to stay
in the headerbar until another contact is selected.

To fix this, make set_shown_contact(null) set the right header title
to an empty string, clearing the headerbar when a contact is deleted.

Closes #128
2 jobs for fix-right-header-title in 6 minutes and 20 seconds
Status Job ID Name Coverage
  Review
passed #1141503
style-check

00:00:10

 
  Test
passed #1141504
flatpak
flatpak

00:06:09