[FEATURE] TYPO3 10 Compatibility

Resolves #109.

Implements #99.

Implements #78

Implements #91.

Implements feature requested in #92.

Fixes #104.

Resolves #93.

Fixes #106.

This reverts commit 90c020a8.
The bug that was fixed with the previous commit has since been fixed in
TPYO3 10.4.1 directly.

This reverts commit ae58804d

`TSFE->'sys_language_uid'` has been deprecated in TYPO3 9 and removed in
TYPO3 10.

Starting with TYPO3 10, `PATH_site` is no longer available. Instead,
`\TYPO3\CMS\Core\Core\Environment::getPublicPath() . '/'` is used.

It seems TYPO3v10 expects extension icons to be SVGs, the png is not
displayed in the backend.