[TASK] Add possibility to resolve a default storage

Before FAL a default upload folder has been retrived by setting
$GLOBALS['TYPO3_CONF_VARS']['BE']['fileadminDir'] . 'user_upload'.
With FAL this even could be placed on a remote system. Furthermore
we do not won't users to fiddle around with path like that anymore.

Each Storage already provides "getDefaultFolder" meant for uploads.
But still, if there are several storages, there is no way to figure
out which storage should be the default one for uploads, if it is
legacy code or just not configurable.

This patch introduces the concept of a default storage as a flag
which can be set within the storage record. The automatic creation
of /fileadmin/ - storage will have this flag already set.

Resolves: #55177
Releases: 6.2
Change-Id: I6684670c5acb023bc26596e9a55ca84bb5c41b40
Reviewed-on: https://review.typo3.org/26945
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack