Break out the text anchors to a plugin

With this, Annotator Core has been freed from (almost) all the
text-specific parts. It has no selector definitions, no anchoring
strategies, no anchor or highlight definitions.

Also no native UI actions to create annotations.

To get the usual functionality, load the TextAnchors plugin, which
 * Recognizes the new selection upon releasing of the mouse button
 * Can create the selectors appropriate for text anchors
 * Has the TextAnchor class (the anchor implementation for text ranges)
 * Has the TextHight class (the highligh implementation for text ranges)
 * Has the two basic anchoring strategies for text (XPath and position)

To get the full functionality, with fuzzy mathing, load the
FuzzyTextAnchors plugin, too, which adds two more anchoring strategies
for the same selectors and anchor type.

Other plugins can now other kind of anchors easily.