Support El Capitan

[chrishunt]

Fixes https://github.com/chrishunt/desktop/issues/28.

We can no longer update the Mac OS X default desktop image because it's protected by the mighty El Capitan SIP. This PR makes it so that we write our desktop image to Application Support instead.

You can test this pre-release with:

$ gem update desktop --pre

[chrishunt]

@fullybaked, @jbrooksuk: Would you mind giving this pre-release a shot whenever you get a chance and let me know what you find?

$ gem update desktop --pre

[fullybaked]

Seems to be working with one exception.

If you set one of your desktop images via the System Preferences, desktop doesn't set that image to the argument passed in, but instead goes back to the system default (in this case El Capitan)

All new desktops created will have the image passed to the desktop command.

To reproduce:

• Create n desktops where n is greater than 1
• Use the desktop command to set the image on all desktops
• Select one desktop and use the OSX System Preferences to change the image to something else
• Use the desktop command again

You will see that the altered desktop is not the image you chose in System Prefs nor the one passed as an arg to desktop.

This could be an acceptable issue though ;) I'm happy to use the gem as is, as I rarely set the desktop image directly, preferring to use this gem

[jbrooksuk]

Any movement on this? :)

[legomind]

@chrishunt the dock restarts, but the desktop image never changes on my machine.

[jbrooksuk]

@chrishunt Version 10.11.3 (15D21) desktop just switched back to the default El Capitan background.