Rework internal Error handler (window) to be a notification agent

The Error handling mechanism (e.g. ER_NewError(), YAM_ER.c) should be reworked to achieve the following things:

1. Introduce a difference between "Errors" and "Warnings" so that YAM might notify the user of things that are of minor priority (e.g. images missing, decoding failure of main mail part, etc.)
2. Rename the whole Error mechanism into a more general "Notification" service/agent thingy.
3. The notification/error window should be changed to contain two areas to present the pending notifications: On the left side a listview with the summary of the pending notifications. On the right a floattext object with the actual message of the notification.
4. Warnings/Errors should be directly differentated via graphical images in the listview. Pending notifications should be bold.
5. Add a functionality to send notifications that won't automatically popup the notification window but show a sign (e.g. in the window bar of YAM) in case minor warnings are shown.