Add option to close Kate when last file closes

Summary: This is a proposal for Kate following the discussion in D15278.

Test Plan:
Open one file in Kate.
In Settings > General options, enable the new option "Close Kate entirely when the last file is closed"
Close the file by any mean (tab button, Ctrl-W, ...)
Kate should shutdown.

Reviewers: Kate, VDG, cullmann

Reviewed By: Kate, cullmann

Subscribers: filipesaraiva, pino, aacid, cullmann, zetazeta, ngraham, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D16169