Clean up the CMake files (and a couple of other bits)

src/imageformats/CMakeLists.txt is now much clearer and more consistent.

Also, removed an unnecessary/unimplemented method from exr.cpp.

REVIEW: 115120

src/imageformats/exr.h

@@ -20,7 +20,6 @@ public:
 
     virtual bool canRead() const;
     virtual bool read(QImage *outImage);
-    virtual bool write(const QImage &image);
 
     static bool canRead(QIODevice *device);
 };