[BUGFIX] Use plugin icon in content element preview

It is possible to register a dedicated icon for a plugin in
ExtensionUtility::registerPlugin(). This icon is used in the
plugin select field, but not for the content element preview
where always a default icon is shown.

With this change, a possible configured plugin icon is used
in content element preview, if the icon is registered using
the TYPO3 icon API.

Additionally the indexed_search plugin now uses
'mimetypes-x-content-form-search' as plugin icon.

Resolves: #97635
Releases: main, 11.5
Change-Id: I8e688c7e4c75f92c35f9f82f32b8434d44829e05
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74697


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>