Templates relating to the core plugin should only be instantiated in the core plugin. This ensures you can't accidentally re-instantiate them when you include core/screen.h or core/window.h (LP: #1066799) (LP: #1066796). Fixes: https://bugs.launchpad.net/bugs/1066796, https://bugs.launchpad.net/bugs/1066799. Approved by Sam Spilsbury, PS Jenkins bot.