Add KFormat::formatPermission

Mostly comes from KIO's KFileItemPrivate::parsePermissions. It is added here so it can more easily reused outside of KFileItem.

It will help in system/dolphin!494 (merged), lowering the memory used by having to call KFileItem::permissionsString even when not needed.

My idea later being having an alternate friendly user way of expressing user permission in KCoreAddons, for instance:
-r----- -> "Readable"
----r----- -> "Readable"
-r---w---- -> "Readable and Writable"...
-----rw-- -> "Admin writable"

Question:

• Maybe there is b better place than in KFormat for this.