Previously the code model would only get updated when the document that
contains the qmlRegisterType changed. If the actual component exported
to QML lives in a different file, any updates to that component would
be lost. With this change we keep track of the header files the
components are declared in and rescan if any of them changes.

This is still not the greatest way to do it as there are a number of
ways to introduce dependencies the system cannot detect. It's better
than before, though.

Change-Id: Ic077c516dca3ac720f78973c84e5e6e91b6a5c07
Reviewed-by: Marco Benelli <marco.benelli@qt.io>