Use qt_subtract_from_timeout instead of duplicating qt_timeout_value

Use qt_subtract_from_timeout instead of duplicating qt_timeout_value

The same function was duplicated 5 times in qtbase, so create one copy
to rule them all and use it also in QtDeclarative.

Change-Id: I4e39a7ee0541ce4fe9710cea344e537ee011bbe9
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>