Simplify convertDomPointToCursorStep method

This ```convertDomPointToCursorStep``` method on ```OdtDocument``` has a parameter that lets the caller provide a function to customise the rounding behaviour. In practice the callers of this method simply use the default fallback of rounding down or provide the same duplicate function to get round up behaviour.

This PR simplifies the interface by only providing an option to either round down or round up when converting to steps. This does not touch the ```OdtStepsTranslator``` interface so any class that might need custom rounding behaviour in the future can still access it directly from the steps translator.
I am slighlty confused.
In the commit message of 71914c795507390108a2e5df164e10a62c6d1caa you write:
"There is also no default fallback to round down behaviour anymore. Callers must specify the behaviour they need."
And most places that call `convertDomPointToCursorStep` got changed to explicitely pass `PREVIOUS`. But not all. And here the parameter `roundDirection` is still marked as optional. Also does the API dox say:
"If a rounding direction is not supplied then the default is to round down to the previous step."
Please help me lose my confusion :)