Add batch actions top bar element

[rocodes]

Status

Ready for review but presumes #2230 which should be reviewed first, will rebase once that's merged

Description

Add Top Bar UI element nested inside main layout, which will hold "Delete Sources" toolbar button (and eventually, other toolbar buttons). UI-only changes towards, #2160

Test Plan

• Visual review
• CI

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration is self-contained and applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[cfm]

Rebased from main to kick the tires. Oddly, GitHub still shows a conflict with main, but we can resolve that in the course of finishing and testing this work.