Fix import of elements and packages

Plugins now install XMI files in addition to libs. QXMIReader tries
to locate xmi files in the same directory of open files and also
in plugins/metamodels directory.

Change-Id: I2fe83596ac0394c21ec29145138bb8810e860770
Reviewed-by: Sandro S. Andrade <sandroandrade@kde.org>