Do some clean up and minor fixes for stability

Fixed a bug when loading a file whose extensions were not present.
Replaced all occurrences of MessageDialog to use DialogUtils instead.

Change-Id: I42a078fbfb6c48177ad1bedfde98d5ef9c416481
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>
Remove caching inside extension items

Whenever the cache is resetted, a timestamp is updated. The next time an
already configured extension item requests a ConfigParameterDescription
it first checks whether it has to fetch fresh data from the DS Service.

Change-Id: Iaf728bca0bbb6607fa204321beed23fa2c43ba32
Signed-off-by: Denis Knoepfle <denis.knoepfle@sap.com>