Add the default example metadata as resources to the demo launcher,
which let's us build the launcher and examples from qtdoc repo
independently without adding metadata from yet another git repo.
The applications-root is stil used to look for additional metadata.

Don't show the demo on the launcher menu unless the binary exists.
Change the metadata so that, if binary path is relative, use the
configured Qt examples path to prefix it. Use the configured Qt
example path also for searching the metadata.

The calqlatr has different name in different Qt version, add both
to support all Qt versions.

Change-Id: Ia59ca8914459126bad309a937a3c671fd7ae5697
Reviewed-by: Ari Parkkila <ari.parkkila@qt.io>