Save/load crate state. Fixes #167.
Remove useless space around "::" in function definitions.

This was done using the following quick’n’dirty script:

    #!/usr/bin/env ruby
    Dir["**/*.cpp"].sort.each do |path|
      print "Formatting #{path}... "
      str = File.read(path)
      str.gsub!(" :: ", "::")
      File.open(path, "w"){|f| f.write(str)}
      puts "Done."
    end

    puts "Finished."

Cf. ticket #87.
Add some fixmes
Disable level editor when the "Enter" button is clicked. Fixes #148.

Use the "Edit" button if you want to edit a level (why do we have
that button, hmmmmm?).