Remove default folder inside default folder.

That folder was added in 12768e35b7f4df9e036b6d2c1b798bdfa597bc04
accidentally, but is not required.
Add empty intro cutscene.

The flare engine is warning about not having an intro cutscene, but
we decided to have none as of now, so this commit will suppress the
warning.
Add antlion_ment sound (silence) and wyvern_critdie (copy of wyvern_die)

This will suppress the warnings, failing to load these sounds.
Move empty initial cutscene to alpha_demo
Merge pull request #301 from stefanbeller/master

some cleanup.