God of Thunder: Add combined levels (one per episode) and more graphics
Secret Agent: Add levels for episode 1
Secret Agent: Add levels for episodes 2 and 3
Vinyl: Remove instrument items and correct a filename

The instrument files are loaded automatically as supp items so they don't need
to be listed separately, as if they were items to be opened in an editor.
Fix crash when double-clicking on folders in the tree view

Also fixes potential crashes when right clicking and viewing item properties.
Make the compiler complain if an enum member is left out of dep2string()