Breaking: #79259 - EXT:t3skin removed

Description

The system extension t3skin has been removed, as all functionality has been migrated into other system extensions.

All ExtJS-related images and css files have been moved to EXT:core.

All other images have been unused for a while now, and have been deleted from the TYPO3 core.

Any direct references to EXT:t3skin now lead to missing styling or image(s).

All installations that use CSS files or images from EXT:t3skin.

Do not use ExtJS styling or images anymore, as ExtJS will be removed from the core.

Other direct references to image(s) in EXT:t3skin should be migrated to have the image(s) in custom extension.