CRM_Extension_Container_Basic - Add mechanism for filtering

CRM_Extension_Container_Basic - Add mechanism for filtering

This function allows one to filter down the list of extensions which are visible.

A few notes:

* The result of the scanning/filtering is stored in cache.  On a typical
  page-load, the system uses this cache -- which happily means that
  filtering shouldn't result in any extra page-load time.

* The extension-system is generally boot-critical.  I could imagine some
  scope-creep around making making the filter mechanism hookable, but that's
  a separate+complicated issue.  Optimistically...  if `array $containers`
  somehow becomes hookable, then one could use the same mechanism to
  manipulate the filters.