UnityShell: enable closing on middle-click, fix scaled decoration sizes, fix theming issues, added scale autopilot tests. Fixes: https://bugs.launchpad.net/bugs/1052821, https://bugs.launchpad.net/bugs/1053225, https://bugs.launchpad.net/bugs/1055605, https://bugs.launchpad.net/bugs/1055609, https://bugs.launchpad.net/bugs/1055610. Approved by Brandon Schaefer.