Add DFI in a server view

[evshvarov]

Introduce DFI assets in server code view:
Pivots and Dashboards
Both pivots and dashboards have folders - would be great to organize it same way
Introduce export of pivots and Dashboards - it usually being exported as xml files in
/src/dfi/folder/name.pivot.xml or
/src/dfi/folder/name.dash.xml