Make _colorbox_doheader() more accessible

Problem/Motivation

_colorbox_doheader() has _colorbox_active() check which works fine for the sites with colorbox on almost all pages. However, sites that want to allow colorbox for certain node type (that are not path specific), it's hard to exclude pages. This is causing unnecessary js/css load on pages that don't have lightbox.

Proposed resolution

Fix the _colorbox_doheader() so that it's possible to add header from other module without hacking colorbox.

Remaining tasks

User interface changes

N/A

API changes

N/A

Data model changes

N/A