libcore|FS: Added methods for iterating all found files

Supports lambda functions.
Unix|Client: Let de::App determine the base path
Resources|Manifest: Also use FS2 to locate files

While FS1 is still used primarily like before, this now allows looking
up the file anywhere in FS2, too. With this change, it is now possible
to launch Doomsday.app on OS X without any arguments (assuming the
IWAD folder is determined via other means) and things will just work:
the needed resources are located within the bundled game plugins.