commit f045452743 introduced the
FocusGained autocommand. Unfortunately, for some systems this might
trigger already at startup, resulting in an unwanted redraw that might
clear the intro screen.
Therefore, add a timer, that enables the FocusGained autocommand after 5
seconds. The 5 seconds is somewhat arbritrarely, but we could change it
later if it turns out to be not useable.
fixes#1817
33a633766d