Filter to be able to change default subnav item easily

[slaFFik]

I'm working with groups and I need t rearrange all items and change the default subnav from any (not necessarily 'home') to any.
The only option I found - is to change its declaration. There is no filters oh hooks in that function, so I'm proposing to implement a filter.
See attached.

[boonebgorges]

(In [4833]) Adds filters and a hook to bp_core_new_nav_item() to allow for easier configuration of the default nav slug. Fixes #3379, props slaFFik