ParagraphLayout: Added TextOffsetAt()

It returns the text offset at a given coordinate, and whether
the coordinate was more to the left or more to the right of the
glpyh center.
TextDocumentLayout: Added TextOffsetAt()

It iterates over the ParagraphLayoutInfos and finds
the one which must contain the location. Untestet.
TextDocumentView: Implemented beginnings of mouse-selection