Clean up theme check in PackageManager with PackageActivators

You can now call `PackageManager.registerPackageActivator` for specific
package types. On `PackageManager.activate` it will call your
`PackageActivator.activatePackages(packages)` with the proper packages.

src/theme-manager.coffee

@@ -17,6 +17,7 @@ class ThemeManager
 
   constructor: (@packageManager) ->
     @lessCache = null
+    @packageManager.registerPackageActivator(this, ['theme'])
 
   # Internal-only:
   getAvailableNames: ->
@@ -38,6 +39,9 @@ class ThemeManager
   getLoadedThemes: ->
     pack for pack in @packageManager.getLoadedPackages() when pack.isTheme()
 
+  # Internal-only: adhere to the PackageActivator interface
+  activatePackages: (themePackages) -> @activateThemes()
+
   # Internal-only:
   activateThemes: ->
     # atom.config.observe runs the callback once, then on subsequent changes.