Don't cut off zoomed font sizes to the nearest integer point size

This caused it to go to 0 when for example displaying size 9 at 10%,
which is an invalid value so it caused the text to revert back to the
default size.

It also caused zooming to sometimes appearing to have no effect. For
example zooming size 9 to 110% would still yield size 9 rather than 9.9.

Task-number: QTCREATORBUG-2744
Task-number: QTCREATORBUG-2745
Reviewed-by: Robert Loehning
Reviewed-by: hjk