Theme manager

This is the first implementation of a theme manager. It’s a rebased and
revisited version of #792.

zonebg/CMakeLists.txt

-# CMakeLists for zonebg/ directory
-#
-# Installs default "zone background" files
-
-FILE(GLOB zonebg "${CMAKE_CURRENT_SOURCE_DIR}/*.png" "${CMAKE_CURRENT_SOURCE_DIR}/*.jpg")
-
-if(UNIX)
-    if(APPLE)
-      	INSTALL(FILES ${zonebg} DESTINATION cockatrice.app/Contents/Resources/zonebg/)
-    else()
-        # Assume linux
-        INSTALL(FILES ${zonebg} DESTINATION share/cockatrice/zonebg/)
-    endif()
-elseif(WIN32)
-    INSTALL(FILES ${zonebg} DESTINATION zonebg/)
-endif()
\ No newline at end of file