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

in order to send a vopStateChangedNotification even if `myInput.value = "something"` is used by any script!

Update dev components version and update tests accordingly

add whitespace after pages in scrolled mode to make sure, it is possible to scroll to last page if it's smaller than viewport

refactored versions of getLastMessage and recordMessages are included for testing purpose and will be moved to iqb-dev-components later.