Remove rounding predicate from step conversion

This removes the option to provide a rounding predicate to decide whether
to round up or down from the convertDomPointToCursorStep method. It is replaced
with an enum that specifies whether to round up or down. The predicate was
removed as all callers supplied the same function to indicate they wanted
round up behaviour and this predicate was being duplicated.

The rounding parameter is optional and defaults to rounding down to the
previous step which is the same as before this commit.
Use core.StepsDirection enum for rounding direction

The rounding direction constants in OdtStepsTranslator overlap with the
existing enum in core.StepsDirection. It makes sense to use that enum here
to specify rounding behaviour in converting DOM points to steps so the
constants have been removed and the enum has been substituted for all
usages.
Merge pull request #759 from satsuper/rounding-cleanup

Simplify convertDomPointToCursorStep method