Add new template function to produce list of activity types

[DJPaul]

Attached patch contains a proposed template function to produce a list of all the registered activity types for use in a <select> element, or as <input type="checkbox">.

When working on turtleshell, I wanted to add filter controls to the activity directory, just like how BP-Default has a dropdown box. Unfortunately, these are hardcoded into the BP-Default templates. We already register activity types, but we don't have a template function to output the data.

My patch implements such a template function and allows choice between rendering the activity types as a dropdown box or as a select dropdown.

[karmatosed]

This rocks! Really cool and offers up flexibility for themes to pick what they want also. I'd like to see such flexibility in other areas so this is a great start I think of letting the template writer pick what they want format wise.

[DJPaul]

Edited description to read: dropdown box or a select dropdown. This patch doesn't do radio buttons, sorry for confusion.

[djpaul]

(In [6502]) Add new activity template function to renders a list of all the registered activity types for use in a <select> element, or as <input type="checkbox">.

• For the <select> output, this is backwards compatible with the "bp_activity_filter_options" action which was hardcoded into the BP-Default theme templates.
• Fixes #4637