Range: add method to get coordinages
Tweaks around enablind/disabling annotating, and showing the adder

Annotator:

 * Enhanced the enableAnnotationg() method with more functionality,
   and added disableAnnotating() shortcut
 * Bugfix: don't forget the reset the selected targets on a failed selection
 * Launch with annotating disabled; enable annotating after scan finished.

TextAnchors:

 * Simplify the calling of checkEndSelection(), and improve coordinate detection
 * on enableAnnotating events, check if there is a selection, and act upon it
 * Remove the mechanism for checking selection on plugin init; this is now
   replaced by the more generic reaction to the enableAnnotating events.
Further tune the Adder button-related behaviors

Annotator core:
 * Introduce the `inAdderClick` property, which is true when an
   adder click is in progress.
 * Modify `onSuccessfulSelection()` method so that when annotating is
   forbidden (`canAnnotate` is false), it does not accept new selections.
   In these cases, it displays a message, and
     returns `false`. The mechanisms calling `onSuccessfulSelection()` are
     supposed to react to this `false` return value by undoing whatever
     visual elements they have created, corresponsing to this selection.
 * Added more documentation

TextAnchors plugin:
 * Started to observe the `inAdderClick` property.
 * No longer care about the `canAnnotate` property.
   (This is now handled in core)
 * Added more documentation