Introduces template hierarchy for top-level theme compat templates

The "top-level" template is the outermost template included by theme
compatibility. As of BuddyPress 1.7, a stack of template names is checked when
BP decides which top-level template to use: plugin-buddypres.php,
buddypress.php, community.php, etc. (See bp_get_theme_compat_templates().)

This changeset introduces an additional level of template hierarchy, so that
themes may provide top-level templates that are specific to the current
component, such as blogs/index-directory.php, or specific to the current item,
action, or status, such as groups/single/index-id-{$group_id}.php. In this way,
BP's top-level template hierarchy becomes more flexible for theme devs, and
more closely parallels the template hierarchy used by WordPress.

Note that this changeset only implements hierarchy for the top-level templates.
Inner template parts may receive a parallel hierarchy in a future version of
BuddyPress.

Fixes #4639

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7212 cdf35c40-ae34-48e0-9cc9-0c9da1808c22