gnome_welcome: Work around unreliable keynav in gnome-initial-setup

The issue with timezone entry occasionally failing appears to be due to widget focus being random. If the focus is already in the search_entry box then pressing 'tab' moves it out and then type_string doesn't enter text there.

Really the widget focus should be consistent, but that is not simple to achieve: see gnome-initial-setup#156.

Work around by clicking on the search_entry before typing.

Fixes: #31 (closed)