This is a subtile change in Verona3

from the top-level iof the JSON where it was places wrongly.

* Adds startPage in playerConfig in vopStartCommand to let the player jump to the previously selected page; this might also be helpful in evaluation use cases where (after the test) the given answers of items at a specific page are expected to be presented

* makes validPages and currentPage optional in playerState for players w/o paging feature

In Verona2 it was mandatary, now it's consecutive:

"made validPages and currentPage optional in playerState for players w/o paging feature"

removed non-standard property unitCount, which was not implemented but enabledNavigationTargets

It is assumed that no response will be reported unless it is valid, so complete will do

Remove vopGetStateResponse; the player should answer the vopGetStateRequest by sending vopStateChangedNotification with all data parts

fixes https://github.com/iqb-berlin/verona-player-simple/issues/3

Add regression test for that.

The implicit form submission had two different negative effects:
- Hitting enter in an input-field could navigate the test-host's iframe to an unknown page
- Bizarre enough browsers click the frist <button>-element in a form after the enter-button got hit in any input-field

Fixes https://github.com/iqb-berlin/verona-player-simple/issues/5