When a plugin is installed in the wrong directory, the class loading
will fail. This patch tries to find the correct directory from the
plugin.info.txt (using the base key) and give a hint to the user on how
to fix this.

This merges the INDEXER_PAGE_ADD and INDEXER_METADATA_INDEX events and
introduces the new string keys 'page', 'body' and 'metadata' in the
event data. All plugins that use INDEXER_PAGE_ADD need to be adjusted to
use the key 'page' instead of 0 and 'body' instead of 1.

In 09edb711 most of the old userstyles were wrongly named. This renames them back to what they were before that change. See also http://www.dokuwiki.org/devel:css#user_styles

* code cleanup in lib/exe/css.php
* renamed 'default' userstyle to 'screen' in config_cascade
* splitted core lib/styles/style.css up into all.css, print.css and screen.css

Attention: $lang['register'] has been renamed to $lang['btn_register'],
anyone using that in any plugin or template should adjust it.