Ensure we quickly show/hide the systray window on startup

Ensure we quickly show/hide the systray window on startup

This is quick enough that no flickering should appear in practice.

We end up doing this because for some reason on Windows (I dug up deep
into the Windows QPA without nailing it down) not showing that systray
window at least once prevents the app object to return from exec() when
the session ends.

It's as if that window would be in some limbo state (neither opened nor
closed) which would prevent quitting. Clearly what we're doing here is a
workaround...

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>