Consider introducing an 'ontagfound' event

[zolkis]

This is eventual future work, coming from #602 and related discussions, to align the usage flow of the API with how NFC actually works (also on Android) - namely, react to a "tagfound" event (optionally, allowing later a "tag released" event, and also allowing a start() method akin to Generic Sensors). Following a "tagfound" event, the tag may be then read and written, instead of installing readers that would provide content on the next tap (and interfere with write, for which scripts currently need a workaround).

Also, the implementation is more complex than needed, because supporting multiple readers (as a consequence of storing filters, which are not any more supported). Also, some internal slots can be removed and the reading algorithm can be simplified.