Handle all Qt command line options (--help-all)

Summary:
This was broken in c5897d0eabf515a82a62429a726819cb60e255ae suggesting
that it might be needed to

fix --help, --version and option errors not being reported on the
terminal if another instance of KAlarm is already running.

Testing suggests that this is not the case, especially since
the printing to the terminal was handled directly by process
in which case it'll never return. I don't really see how the
printing could be stopped unless stdout or stderr is overriden,
which doesn't seem to be the case.

This is the same as how KDE builtin options are currently handled.

Also remove now unused EXIT command.

Reviewers: djarvie, lbeltrame

Reviewed By: djarvie

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28704