Add "osx" click-to-focus mode.

- Requires cinnamon-desktop wm schema update.
- Clicking on a window focuses and raises it, but does not
  pass the button click event to the window.

Extra notes, questions:
- Haven't tested thoroughly...
- testing, more testing...
- enable mode by:
     gsettings set org.cinnamon.desktop.wm.preferences focus-mode 'osx'
- escape it via cinnamon settings
- still needs cinnamon patch for settings