bp-legacy: Messages loop missing table header action

[r-a-y]

In the messages loop, there is an action to add an extra table column - 'bp_messages_inbox_list_item':
​https://buddypress.trac.wordpress.org/browser/tags/2.2.1/src/bp-templates/bp-legacy/buddypress/members/single/messages/messages-loop.php#L64

But, there isn't a similar hook in the table header:
​https://buddypress.trac.wordpress.org/browser/tags/2.2.1/src/bp-templates/bp-legacy/buddypress/members/single/messages/messages-loop.php#L25

This means attempting to add a table column will not work properly.

Attached patch adds a hook called 'bp_messages_inbox_list_header' in the <thead> of the template so plugins can properly add a table column.

This bug also exists in bp-default.

[tw2113]

Updated the patch to include a phpdoc for the hook.

[r-a-y]

Anyone got any qualms with the hook name before I commit?

'bp_messages_inbox_list_header', 'bp_messages_inbox_list_head' or something else?

[boonebgorges]

'bp_messages_inbox_list_header' seems fine to me.

[tw2113]

Do we have a different spot with a similar hook, or perhaps a hook missing that needs patched as well?

"header" is fine in my books as well.

[r-a-y]

In 9840:

bp-legacy: Add missing 'bp_messages_inbox_list_header' hook.

This is to mirror the 'bp_messages_inbox_list_item' hook to allow plugins
the ability to add an extra table column in the messages-loop.php template.

Props tw2113.

Fixes #6328.