Screen share fixes

These This series fixes a minor issue with screen sharing module that wouldn't start if there's no pointer available which is no longer the case from commit e825fe38 (the pointer is present only when we detect movement). Choosing an output to share is made with some helpers commonly found in shell utilities, so this series first group them together and re-uses them when needed, in the screen share module, and in desktop/kiosk-shell.

I've include two more fixes with it:

• a fix to #298 (closed), to Add a compositor destroy listener
• a fix `runtime error: shift exponent XXXXX is too large for a 32-bit type 'int' -- bit shifting SHM formats

And one more to document --no-config option for starting screen-share at weston/compositor start-up.