Allow extensions to define CSS themes

= Context =

We are looking to improve CiviCRM's UI along two fronts – changing both the visual language (with a more modern appearance) and the CSS conventions (with a more standardized, Bootstrap-based markup – enabling easier integration and training). During the interim period (while the new visuals/conventions mature), we need to ensure that:

• (a) some sites (with limited needs/screens/use-cases) can go live
• (b) the existing look and feel doesn't get compromised, and
• (c) existing extensions/customizations continue to work.

= Architecture =

There will be two visual languages (themes), Greenwich (the current look-and-feel) and Shoreditch (the new look-and-feel). These themes are defined by way of hook_civicrm_themes.

Any theme should define two main CSS files, "css/civicrm.css" (which provides styling for the current markup using the crm-* coding convention) and "css/bootstrap.css" (which provides styling for new markup using the http://getbootstrap.com/ coding conventions).

Any request to add a CSS file will be passed through the theming layer. If the theme does not provide the file (via hook or naming-convention), then it will be loaded the same as today.