Move Qt examples to a separate repository

Move the Qt examples out of the individual git submodules into a single qtexamples.git submodule.

Advantages:

• Examples are not limited anymore to the Qt modules that are part of the repository.
  That is, e.g. qtdeclarative examples can also make use of QtQuick.Controls, qtnetwork examples could use Qt Quick ...
• Users can easily explore all examples by cloning one single repository.

Disadvantages:

• Changes in the Qt API that break an example are not blocked by the CI of the respective repository.
• Examples are often also the source of snippets in the documentation. These would need to replaced by dedicated snippets.
• Examples cannot be linked anymore by module documentation. Instead, we should arguably have a 'qtexamples' qdoc module, too.
• It creates an artificial boundary between playground modules outside of Qt releases, and modules inside Qt.