bp_core_new_nav_default() function is useless

[imath]

Reviewing 8139.current-action.patch about #8139 I've found, using bp_core_new_nav_default() function is making the BP Component hardcoded Default subnav unusable.

To reproduce, add a new bp-custom.php file with this code:

function bp_activity_component_new_default_subnav() {
	bp_core_new_nav_default(
		array(
			'parent_slug'     => bp_get_activity_slug(),
			'screen_function' => 'bp_activity_screen_mentions',
			'subnav_slug'     => 'mentions'
		)
	);

}
add_action( 'bp_init', 'bp_activity_component_new_default_subnav', 2000 );

When you go to the Activity user's primary nav, you will reach the mentions subnav. But then the just-me subnav can't be reached anymore (even if you add it manually to the URL).

I've explored the bp_core_new_nav_default() function and I have started to build a patch. It needs to be completed for components != Activity.

Maybe it's a bit late to include it for 8.0.0. But I'd like we talk about this ticket at least before eventually punting it.

[imath]

We need more testing about this change.

[imath]

Let's do it in 10.0.0 instead

[imath]

Let's try to progress on it during 10.0.0 dev cycle.

[imath]

Thanks to our canonical URL management, the patch is less complex: we actually just need to make sure the default subnav slug is added to the corresponding wp_admin_nav item.

[imath]

In 13093:

Make sure bp_core_new_nav_default() updates the previous default nav

• The previous default nav item needs to have its slug added to its link property.
• The default subnav slug needs to be added to the URL used by the corresponding wp_admin_nav item.
• The new default nav item needs to have its slug removed from its link property.

Fixes #8471