dev.html: scan() immediately after initialization
Enhance protection agains double annotation

 * Rename the field 'ignoreMouseup' to 'canAnnotate', and invert the semantics
 * Provide a method to set abovementioned field, which also published an event,
   anouncing the new setting
 * Update the text anchoring plugin to observe the new setting

The concept is that all anchoring mechanisms observe the 'canAnnotate' flag,
and nobody is supposed to call 'onSuccessfulSelection()' when it's set.