ScreenSaver: use ceilf instead of ceil

... to restrict to single float precision. This is a little pedantic and silly
but bare with me for the sake of consistency.
ScreenSaver: Just for good measure allow 4 lines of text here.

... just in case a long translation needs an extra line to fit
(in English 3 is enough) and because we've got enough room next
to the screen image to fit another line of text anyway.
ScreenSaver: Set target for turn off display checkbox

I noticed while testing the nightly on vmware which, unlike vbox, includes
a video driver with DPMS support that this checkbox wasn't working.
This should fix it.