Adjust VC++ projects to fit latest image handling changes.
Let the release package script create a 7-zip package too.
Fix #3965: Application getting out of focus when closing inspectors

At least in Windows some other application gets the focus after closing
a TransientWindow.
Add a text entry box to the LightInspector's colour picker.
Disable warning about virtual inheritance in VC++.

The way this is set up is intended and working correctly, the warning is
merely an information and can be ignored.
Improved support for Dark Mod standalone

- Added new .game file for the Dark Mod, which does not use a "/base/"
  subdirectory under the main engine path. This should remove the need for
  setting the root directory as the engine path.
- Modified Linux code to only look in a home-directory engine path if the
  "prefix" game file property is non-empty. Dark Mod maps are now saved into
  the main installation maps directory on Linux.