Fix MV3 on Firefox for Android

[Samq64]

Resolves #6976
Resolves #7461

Changes

Sets the viewport on the popup and checks if chrome.contextMenus is defined before using it since it errors on Firefox for Android now.

Also removes some weird icon handling I added back in #4810. Now it always uses the icons set in the extension's manifest.

Reason for changes

The popup wasn't scaling correctly and after MV3 the extension completely broke. It's also nice to be able distinguish Git versions even if they're stable.

Tests

Tested on Andorid Firefox Beta, desktop Firefox and Chromium. The viewport doesn't seem to change the popup on Desktop and Firefox works on Android again including Scratch notifier.

[Samq64]

To test on Android:

1. Install ADB and web-ext
2. On Android install Firefox beta
3. Enable the developer options by repeatedly tapping the build number in Android OS settings
4. Enable USB debugging in the developer options (still in device settings!)
5. In Firefox beta enable remote debugging via USB
6. Plug the Android device into the computer and tap allow every time it asks
7. Open a terminal in the Scratch Addons directory
8. Run adb devices
9. Run web-ext run -t firefox-android --adb-device ABC123 --firefox-apk org.mozilla.firefox_beta with ABC123 being the device ID from the previous command
10. The extension should install. Tap OK.
11. If it doesn't, completely close Firefox Beta and run the command again.
12. If a purple message appears it has to be dismissed since it steals all focus.

To inspect the extension:

1. On desktop Firefox open about:debugging
2. Click enable USB debugging
3. Click connect on the device in the sidebar
4. If it doesn't show up restart desktop Firefox
5. The extension card should show up similar a desktop one.

[BroJac5246]

On Android install Firefox beta

This also works on regular Firefox, you just have to change the package name in the web-ext command to org.mozilla.firefox.
They need a better way to do this than using ADB but at least it works...

Anyhoo, can confirm the extension now works (Firefox for Android Stable 126.0; Android 14 QPR2 on Google Pixel 7a). I can't test everything but the extension loads correctly and the addons I try seem to work correctly (at least as well as they can on mobile).

The initial permissions page didn't display correctly in either orientation but was usable exclusively in portrait mode. I don't know if that's a new development, however. Either way, we should get this out as soon as possible as long as it's confirmed this won't break anything on desktop browsers.

[WorldLanguages]

Thanks for figuring out how to test on mobile Firefox, I wasn't that lucky as I got some weird error when trying to run the web-ext command.

[WorldLanguages]

Could this cause some issues on desktop? 🤔 The size of the popup has frequently created problems.
Maybe to be on the safe side we should add this dynamically with JavaScript if the user agent matches mobile Chromium or mobile Firefox?

[Samq64]

I haven't noticed any problems so far and Vue 3 also adds it but it's better to be safe I guess.

[mxmou]

Unless it's handled differently in extension popups for some reason, the viewport tag has no effect on desktop.

[WorldLanguages]

Then let's assume this didn't break anything unless we've got proof otherwise