INSTALLABLE_DIR improvements for combined applications

[permcody]

This implements an improvement to the INSTALLABLE_DIRS feature already present in MOOSE so that submodules can have resources automatically included in the top-level application's installable applications.

• If neither the parent or sub publishes a list of install dirs, only the parents resources are installed.
• If only the parent publishes, only the parent resources are made available.
• If the sub publishes a list but the parent does not, the sub's resources and default resources (tests) from the parent are made available.
• If both the parent and sub publish resources, all are made available.