main: do not trigger FocusGained on startup
main: do not trigger FocusGained on startup

commit f045452743736d53c7471bfb4698fd48569dc1e0 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

chrisbra/vim-airline,yzhou61/vim-airline,rchicoli/vim-airline,bling/vim-airline