The idea is that if you have your cursor at the begining of a Writer
document, and you scroll down a lot, then search, then it's annoying
that search jumps back to the start of the document for the first hit.

Add an optional way to provide what is the starting point of such a
search, so we can have "when nothing is selected, then search from the
top left corner of the visible area".

No UI yet to enable this, but available via the UNO API.

Change-Id: Ibcf3a5f2eeba1372b1dfe8474081e6591a6e0134