Add template hierarchy support into BuddyPress

[DJPaul]

WordPress' ​template hierarchy allows it to use specific template files for specific pages, or fall back to a default.

The attached patch is a first-pass of introducing the same to BuddyPress, built against the trunk. It has been taken out of a custom plugin and it has a designed limitation that it won't affect the main page templates which are always included by bp_core_load_template().

Most of the BP-Default theme uses bp_core_load_template() to load a "main" template file which then includes an appropriate secondary template to build the page; the latter is where I decided to patch.

[DJPaul]

v1

[DJPaul]

May I have some feedback on this please? I'd like to put it in trunk. Specifically whether to fetch item IDs for BP Core components; at the moment it uses part of the URL to keep things compatible with all plugins.

[DJPaul]

After some discussion with John on IRC, I'm going to look to see if I can make a forwards-compatible patch to enable this to work with WP 3.1. The plan is to retain existing behaviour if not on WP 3.1.

[DJPaul]

Bumping this to 1.4 as we'll need to ask for a filter to be added in WP Core to locate_template().

[DJPaul]

​#WP13239 looks relevant

[DJPaul]

Closing in favour of #3741, which has the up-to-date approach for this sort of thing.