Plugins Pack - rev.6 addon by Okko

Changelog:
- Added Quake II plugin, based on the QRevPAK Wii gaming engine by Izhido (https://wiibrew.org/wiki/Q2Rev)
- Added Serpent Riders (Heretic, Hexen) plugins, based on the Wii ports by Arikado (https://wiibrew.org/wiki/Heretic_Wii, https://wiibrew.org/wiki/Hexen_Wii)
- Replace the comment "#QRevPack Quake 1" with "#QRevPack Quake 2" in platform.ini, for reuse of the magic number in the related Quake II plugin
- Added Heretic and Hexen magic numbers to platform.ini
- Added box covers of the three new games (Quake II, Heretic, Hexen) included in this addon 
