Major cleanup of templates and code + delayed loading of album art

* a6fe517 removes the deprecated `from lib.*` and `import lib.*` code. Since the lib folder was added to the path, this method was deprecated.
* 3fef151, cce9870 cleanup HTML and plugins. Some plugins were outdated and duplicated in the code. I sorted out what was global and what was interface specific (back in the days, when Headphones supported themes).
* 5eef5a5, d04dff9 and 298daf9 implement load-on-view behaviour for album art images on the front, search and upcoming page. Images are only requested when they are actually shown. Especially on the search page, this saves A LOT of useless requests. Further more, there was a bug which continuously loaded the CCA image on error, even if that image did not exist.

Most of the changes are template related. I have checked most of the pages, dialogs, checkboxes, alerts and so on, and they seem to work, both in Chrome v37 and IE11.0.