AGOS: Added detection for Simon the Sorcerer 1 CD32 demo

I have verified that the demo is completable, though it doesn't
actually seems to end. You're just told that there is nothing more
for you to do.
AGOS: Fix invisibility in Amiga CD32 Simon the Sorcerer 1 demo

This probably affects the full version as well, but I haven't
verified that. Either way, the old code was obviously wrong while
the new makes it look right to me.
COMMON: make XMLParser::loadStream() fail when stream is null

Some users of this method relies on it to fail when
an invalid stream is passed to it
(E.g. VirtualKeyboard::openPack).
SCUMM: MANIAC V1 Demo: GF_DEMO was not being set
Merge pull request #535 from segrax/FixDemo

SCUMM: MANIAC V1 Demo: GF_DEMO was not being set
DOCS: Add CONTRIBUTING.md

This will be displayed on github as "guidelines for contributing".
Merge pull request #534 from skristiansson/pr/loadstream

COMMON: make XMLParser::loadStream() fail when stream is null
COMMON: Remove executable bit