Tool tips from parsers issue

As reported here:

http://fifesoft.com/forum/viewtopic.php?f=10&t=597

"If a keyword is the last word of a line followed by a line break, then the tool tip window will even pop up if your mouse is a lot more right of this word but on the same line."

This occurs because we're using View#viewToModel(Point) to determine a document offset at the mouse position. viewToModel() returns the closest offset. We need to change our technique to return null if the mouse isn't actually over a word.