New round of bugfixing

Welcome the newest batch of fixes. A huge ThankYou to The Rushin' Russian for his detailed reports. Due to ongoing problems this update is again only available for Windows and Linux. OSX users will hopefully receive all updates by tomorrow.

Fixes are:

• Fixed black screen in Arcade Survival and Coop Survival modes
• Fixed subway being destructible by explosive weapons
• You can now leave the turret at storage hall alarm in Harbor level manually using at the movement buttons
• Improved quest scripting at the Lighthouse in the Harbor level to avoid getting stuck
• Improved savegame serialisation for Harvester boss to fix Save/Load exploit
• Improved savegame serialisation for Final Boss, stage 2
• Improved several other minor savegame issues
• Fixed large decoration graphics not showing up in Weapon Upgrade screen