Separating text selection and the adder button

This change cuts the Adder plugin in half:
 * The core adder button functionality remain in the Adder plugin.
 * Support for annotating text selections goes into the TextSelector plugin.

The goal is to make it possible to use the adder button with different
kinds of selections, too.

Some communication is necessary between the two plugins. I used
API extension of the core Annotator instance.

 * `annotator.ignoreMouseup` is set by the Adder plugin, and is read by
   the TextSelector plugin, in order not to trigger false selections
   by clicking on the adder button itself.
 * `annotator.selectedSkeleton` is used by the TextSelector plugin
   (and later, by other selector plugins) for storing the annotation
   skeleton prepared based on the selection. The adder button takes the
   skeleton for here, and uses it to create the actual annotation.
 * `annotator.onSuccessfulSelection()` and `annotator.onFailedSelection()`
   methods are triggered by successful and failed selection events detected
   by the TextSelector plugin. The Adder plugin reacts by showing and hiding
   the adder button, respectively.

Other than these integration points, the two plugins are independent
of each other.