qmlcachegen does not remove .qml and .js from resources

QTBUG-73669 silently changed the behavior of qmlcachegen and removed all traces except of the help output.

Before the change, qmlcachegen removed .qml and .js files from resources and generated precached .cpp files for them instead.

After the change, the --filter-resource-file does not filter anything despite its name and even help output.

We're using the compiler especially to remove the plaintext sources from the binaries. This was our reason for the commercial licenses when the compiler was still not part of the open source versions of Qt.

Please re-add the functionality to remove the plaintext sources, update the parameter description and add a section to the documentation about the behavior change starting with Qt 5.14.0