Bug 1383995: Move plugin files into the /theme directory

Theme files for plugins can now live under the plugin's directory or under
the theme's directory. Moving the files for the core plugins to /theme
should make life easier for theme developers by putting them all in one place.

Change-Id: Icff863c46c80ed2d6be07cd49404bc0b7f5fc4c0
Merge "Bug 1383995: Move plugin files into the /theme directory"