[FEATURE] Enable automatic file import for extensions

As preparation for the new distribution (introduction package ...)
handling extensions should be able to provide initial file imports.
These are then imported to fileadmin/<extensionname>
upon installing the extension.

The files have to be located in <extension>/Initialisation/Files/*.

The files are imported only once (even if you reinstall the extension),
execution state is saved via the core registry.

Resolves: #51466
Releases: 6.2
Change-Id: Icbf63227aa8bbb9fcfb0335bfa5ae92b549a94ba
Reviewed-on: https://review.typo3.org/23427
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn