[WIP] Refactored plugins

## TODO:
- [x] **TEST!**
- [x] Make html.tpl working
- [x] Make sending to subscribers from subscribers page working
- [x] Check if auto sending works
- [x] Header message is not working anymore
- [x] refactoring of modvars if plugin is active
- [x] proper handling of modname
- [x] Fix Clip plugin issue.

## Changes:
- Newsletter plugins are now located in `NewsletterPlugin` folder
- Newsletter plugin template folder is now named `plugin_config`
- The plugin modvars are now named like this: `plugin_Newsletter_NewsletterPlugin_Clip` (was `plugin_Clip`). This looks a bit confusing, but is necassary.
- Newsletter plugin special parameters (like clip has) are now handled by the `setPluginVar, getPluginVar, delPluginVar` methods of the abstract class.
- Added a new property for Newsletter plugins: `description`. This is shown on the plugins config site.
- Newsletter plugins now can be located in other modules (will fix #47, ***test case for Clip can be found at https://github.com/cmfcmf/Newsletter/tree/test-plugin***)
- All Newsletter plugins now extend the new `Newsletter_AbstractPlugin` class (will fix #131)
- Moved title to plugin classes (will fix half of #126)
- Fixed some multilingual problems (will fix #83)
- `html_X_items` are now located in new `output/items` folder.
- Renamed DBObject methods, refs https://github.com/zikula/core/blob/master/src/docs/en/dev/upgrading_for_developers.rst#dbobject