[Preferences] add option to set Ctrl-Q behaviour (Quit or nothing)

Motivated by #695 which targets broader task, Ctrl-Q behaviour is one
that stands out. Accidentally pressing it instead of Ctrl-W can have
disasterous consequences or is annoying. Other browsers provide either
generic keyboard shortcuts configuration (Opera) or an extension/plugin
to disable the behaviour (FF).

For the record of the usability demand:
https://code.google.com/p/chromium/issues/detail?id=59621
https://bugzilla.mozilla.org/show_bug.cgi?id=550559