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

Summary:
This was broken in c5897d0e 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