Migrate Clippings to Manifest v3

[aecreations]

Firefox will be implementing Manifest v3. Unlike Google Chrome, it will support Event Pages which is similar to background pages, except that it won't be persistent - it may terminate after a period of time.

Clippings needs to be modified so that it can handle unplanned termination of the background script. This will involve a massive overhaul of the Dexie-based Clippings backend architecture, as extension pages hold references to the background script (via browser.extension.getBackgroundPage(); this should be changed to use WebExtension messaging.

More info on Firefox Manifest v3:

• Manifest v3 in Firefox: Recap & Next Steps: https://blog.mozilla.org/addons/2022/05/18/manifest-v3-in-firefox-recap-next-steps/
• Manifest V3 migration guide: https://extensionworkshop.com/documentation/develop/manifest-v3-migration-guide/

[aecreations]

Seems like there will be a grace period before Manifest v2 is deprecated. From the Mozilla Add-ons Community Blog:

MV2 will continue to work. We have not made decisions about MV2 deprecation yet. We want to see what the response and adoption of MV3 is from the community next year and will then make decisions about the future based on that.

https://blog.mozilla.org/addons/2022/10/31/begin-your-mv3-migration-by-implementing-new-features-today/comment-page-1/#comment-227534

[aecreations]

The browser_style key in the extension manifest is deprecated in MV3, starting in Firefox 115:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Browser_styles#manifest_v3_migration

[aecreations]

Auto-incrementing placeholders are incorrectly reset when the background script terminates. This is because the aeClippingSubst library doesn't save the state of those placeholders into extension storage.

[aecreations]

The Clippings menu items for clippings and folders are duplicated every time the background script is restarted.

[aecreations]

Fixed a regression bug: issue #455

[aecreations]

Need to verify that backup notifications will still appear, especially the first-time backup notification.

[aecreations]

Verified backup reminder notifications.