the workspace selection dialog

ReopenableContentProposalAdapter extends ContentProposalAdapter
instead of ContentAssistCommandAdapter as the latter expects
a WorkBench to be loaded.
This also forces ReopenableContentProposalAdapter to reimplement the
FieldDecoration logic in ContentAssistCommandAdapter.

Tests are deactivated because of Bug 540441 and Bug 275393.

Change-Id: Idbb09dfde30f79c3a723511d75a9bfe21e8740ee
Signed-off-by: Fabian Pfaff <fabian.pfaff@vogella.com>