Support to enable/disable the RTC integration in the UI

On the ProjectInfoScreen there is now a dropdown list for the
plugin.its-rtc.enabled parameter which offers the values true,
false, enforced and INHERIT. Project owners can change this parameter
and save it. If the RTC integration is enforced by a parent project
the dropdown list is disabled.

Change-Id: I596b137bea044e837ce3d2caa9f2c3c7ebed02a2
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>