Address issues with VSIX item publishing projects and FUTDC

[tmat]

Workaround missing dependency publishing feature in the VS SDK.

Introduces the following convention: When a project reference publish outputs (i.e. more than standard build outputs) need to be packaged into a VSIX,
the project should define VsixItemsOutputGroup target and include it in ProjectRefrence.IncludeOutputGroupsInVSIX.

VsixItemsOutputGroup target may gather all the publishing items to be packaged into the VSIX using PublishItemsOutputGroup target,
but should not depend on Publish target (i.e. invoking VsixItemsOutputGroup target should not trigger actual publishing).

A custom target run before CreateVsixContainer will take care of publishing for all ProjectReferences with VsixItemsOutputGroup.