Split compound item implementation to 2 steps

[sohocoke]

Step 1. just take snapshots, initiated from an overlay control specific to compound items (or just browsers?)

• takes care of the 'pinned' sites
• manually deals with 'working progress' pages

Step 2. expand scope to specify capture type in compound item entry in workspace.

[sohocoke]

Proposal: for compound viewports, present:

• 'Snapshot' / 'Snapshot in workspace...'
• 'Track' / 'Track in Workspace'

Move tracking out of scope of first release.

[sohocoke]

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