So we can use the new RTF import for clipboard pastes in Writer without
inserting text content to the end of the document only.

Notes:

- SwXText::insertTextPortion: the MovePara() call is removed, as all it did was
trying to move the cursor beyond the end of the document.
- SwRTFReader::Read: the double fake paragraph insertion / deletion is
motivated by the ODT filter.
- RtfFilter::filter: if TextInsertModeRange is not passed, then the behaviour
is not changed.

v2:

- added missing @since tags
- added insertTextContentWithProperties() method
- removed unused appendParagraph() method

Change-Id: I24cddb00a78e3b798e7d88764e59e6a77a6e98a4
Helped-by: Michael Stahl <mstahl@redhat.com>