Clean up the interface between Adder and TextSelector cleaner (1)

Earlier, the Adder and the TextSelector plugins communicated mostly
using fields on the core. This is not how this is supposed to work,
so now I replaced three of those fields with events.

These fields are now gone:
 * `onSuccessfulSelection`
 * `onFailedSelection`
 * `selectedSkeleton`

The only remaining ugliness is the `ignoreMouseup` field.