AGOS: Give each MIDI track its own loop flag

This is needed because the MIDI resources have their own loop flag
that overrides the global one. This makes a difference for games
that use MIDI both for music and sound effects, such as (limited
to?) the floppy version of Simon the Sorcerer 1.
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: Fix bug #2034 - MANIAC V2: Early Collision with Green Tentacle
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
Merge pull request #533 from segrax/master

SCUMM: Fix bug #2034 - MANIAC V2: Early Collision with Green Tentacle
Merge pull request #526 from eriktorbjorn/agos-midiloop

AGOS: Give each MIDI track its own loop flag (bug #6665)
I18N: Update/improve Dutch translation
I18N: Fix missing translation entries

The last translation entry of a .po file was not properly handled.
I18N: Update translations data file
I18N: Support multiple .po files for same language
I18N: Add support for language names translation