Sidebar changes before the sidePanel migration/merge

[fregante]

• Extracted from #7228: chrome.sidePanel POC 2 #7266 (comment)

The reasoning is that the API is a bit confusing and that shimming the sidePanel version into it might make it hard to reason about. MV2 and MV3 sidePanel have different execution paths (controller in content script vs controller in background)

Also some changes are required for the MV3 version, but can be done outside that PR.

Suggested changes:

• Use timestamps instead of renderSequenceNumber
• Review logic in rehydrateSidebar (it does too much)
• extract updateSidebar from showSidebar