Re-introduce option to toggle FancyMainWindow auto-hiding

This reverts commit 1541fbd2ba8e9d3c12ebdea74453e7e597d961d5
in parts. The title bars can now always be shown (the same
state as the old "Unlocked" state) or not always be shown
(the previously "new" behavior).

The default is now "Always shown". Hopefully that makes hiding
the titlebars a conscious (usually one-time) activity
that's not forgotten.

Change-Id: Id31fb193b2eb6376000c845d4a61194e2ed33129
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>