QtNFC: revisit and update examples

The following issues with examples are identified:

• As we now do not have any support for desktop platforms, the examples UI need to be updated to look good on smartphones. Currently ndefeditor application looks ugly when loading large image and/or creating multiple data fields. Note also that the UI expands to the right and some controls become unavailable (see screenshot).
• The ndefeditor application hangs with a black screen when trying to load a message from incompatible format (not a previously saved NDEF message, but random file).
• The annotatedurl example does not work when we turn on NFC after the application startup. On my phone the default NFC-processing app handles the NFC tag instead. This can probably be solved by subscribing to adapterStateChanged() signal and calling startTargetDetection() when the adapter is enabled. The example description has to be adjusted accordingly.
• It might make sense to extend the annotatedurl example to use QNdefFilter to verify the incoming message. The QNdefFilter class currently lacks the usage examples (be that unit tests or example applications).