Summary:
If a user disables the mpris plugin there's no point loading anything in
the client page.

Also split the mediaSessions shim into a separate option as it's much
more invasive than the regular mpris controls.

For now the latter is disabled by default. It breaks some sites, we
can't afford to do that and we need to put a fix for that immediately.

We can revisit it later.

Test Plan:
Ran media-sessions API page
With extended mpris on I got thumbnails
With it off I still had the classic video control

Reviewers: fvogt, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13628

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

We were only looking at its children but we may also get an audio/video element added directly

Differential Revision: https://phabricator.kde.org/D13378

Chrome syncs installed extensions across computers and will install it also on Windows, macOS and other unsupported platforms.
The extension doesn't work there and showing settings for features that don't exist makes no sense.
The "About" tab is still fully accessible.

Differential Revision: https://phabricator.kde.org/D13326

Chrome syncs installed extensions across computers and will install it also on Windows, macOS and other unsupported platforms.
This would yield a "Make sure p-b-i package is installed" on startup the user can do nothing about.

Differential Revision: https://phabricator.kde.org/D13325

In case the browser takes forever to start or user installs the extension manually without clicking the SNI.

CCBUG: 394673

Differential Revision: https://phabricator.kde.org/D13210

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"