Temporary asset path as distinct vhs folder inside of typo3temp/assets/

[tastendruecker]

https://github.com/FluidTYPO3/vhs/blob/c962d0dce84ad9b1cb2091ba716befab98be34ec/Classes/Service/AssetService.php#L346

Would be nice if the vhs assets were stored inside a distinct folder within typo3temp/assets. Two benefits:

1. No more need to check for TYPO3 version in AssetService->getTempPath()
2. Back-end users could manually remove the asset files using the Install Tool

The directory for the temporary assets just has to be defined within ext_emconf.php like e.g. 'createDirs' => 'typo3temp/assets/tx_vhs',