Heavily rewritten API:

- Extensions can now be tested (used while extension is being installed)
- Flexible filter system added which is database-based
- All "theme-extension-related" functions are now moved to
  inc/libs/theme_functions.php to follow more the "inoffical" guidelines:
  + Write in extensions!
  + Use the new filter system!
  + Write your own functions ALL_UPPER_CASE to avoid confusion
    with php_internal_functions!
- This is a half-finished update, I will change later more places to the
  filter system.
- THINK: Do we need to externalize the session management into an own extension?
- inc/theme-manager.php is now deprecated
- Dummy include file inc/theme-dummy.php will be loaded if no extension "theme"
  is loaded which returns values for default theme.
- Minor fixes/improvements