DTE2 members to implement in new Extensibility Model

Per issue #352 my extension may not be able to use await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync(); and therefore not be convertible to the new Extensibility Model (EM).

I only use that method for accessing some DTE2 members and its use is the reason that my extension is in-proc. If the EM could implement the following DTE2 members my extension conversion can continue (and be out-of-proc.)

• DTE.ExecuteCommand("File.Close") and DTE.ExecuteCommand("File.FormatDocument") -see issue #337.

• `DTE,StatusBar

• DTE.ExecuteCommand() -used in a number of places (e.g. "EditorContextMenus.FileHealthIndicator.RunCustomCodeCleanup") multiple times per command.

How they are implemented (e.g. in their own class, as extension methods, etc.) is of no concern, nor is it a concern if any of them have to use await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync(); as long as they return on the calling thread (or any thread, so long as execution can continue and possibly invoke DTE.ExecuteCommand() multiple times.

Start by reviewing issue #352 and the related issue #337, then trace the new Extensibility Model entry points for DTE access. Check how DTE.ExecuteCommand and DTE.StatusBar are expected to behave, including repeated command calls and thread continuation. Done means the listed DTE2 capabilities are available to an out-of-proc extension with the requested execution behavior.