Check for and notify about "Disabled Items" for Ribbon and RTD Server

The COM helper add-ins used by Excel-DNA for Ribbon support, and in some cases the RTD Server, can be marked as "Disabled" by Excel. This happens most often during debugging, if the Excel process is terminated at some critical times in the execution.

It's possible for Excel-DNA to check or notify the user if there are entries in the "Disabled Items" when loading these helper add-ins. We should investigate this further.