Better to use an instance with a doc service specified

...to avoid detection of filter from other services, and to
additionally check for filters other than the preferred
by the type (see SfxFilterMatcher::GetFilterForProps).

Change-Id: I64609b32f6aac7553b4b6b96db88acfcad0ce000
Base the convert-to detection on the actual service

There is no need to run a type detection on the input
file (for a second time!), just to get the document service.

This also simplifies the conversion of some generic formats
like HTML and RTF, using Calc export filters. Previously
setting both input and output filters was required. Now it's
enough to set the input filter, or simply use --calc.

Change-Id: I825e0fe3e7368d6649acb434b13de53a4523064a