sourcearea plugin availablity

[Garry Yao]

With preset-basic build profile, the editor is built without the sourcearea plugin, this causes a trouble on some samples that it fails to demonstrate the subject, e.g. fullpage and various output samples.

[Frederico Caldeira Knabben]

With now, presenting a message like this is more than enough:
"To fully experience this demo, the x, y and z plugins are required. These plugins are not available on your editor installation.". And that's it, nothing else.

We don't need over-complications because of this at this stage.

[Garry Yao]

Opened dev branch t/9639b for review, it added the compliance check for sample pages, prompt with a warning if necessary.

Based on the branch proposal, a meta is required to indicate a sample that requires source view capacity:

<meta name="ckeditor-sample-tags" content="output">

The tags can be extended later when needed.

[Garry Yao]

Fixed on master with:

​https://github.com/ckeditor/ckeditor-dev/commit/3f21dcc878026f849784f7d568d35e1ab872b2cc

Opened #9644 in seeking for a solution.

[Piotrek Koszuliński]

We've done two mistakes:

1. samples.js should be included only in samples that need it.
2. Simplified meta name (and logic) should've been proposed "ckeditor-sample-required-plugins"

[Garry Yao]

Reopened t/9639 for review.

[Piotrek Koszuliński]

R+, but please fix coding style on commit.

[Garry Yao]

Re-fixed on master with:

​https://github.com/ckeditor/ckeditor-dev/commit/a16ec5b6bc0a51a83ad7f88be3af903749b84493

[Frederico Caldeira Knabben]

The fix doesn't do its job, other than still being over-complicated. Just replace sourcearea with abc on the new meta and no warning is shown.

I'm taking this over.

[Frederico Caldeira Knabben]

Pushed t/9639.

[Frederico Caldeira Knabben]

Fixed with [70db2b1].