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:
- 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.)
- Rename the whole Error mechanism into a more general "Notification" service/agent thingy.
- 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.
- Warnings/Errors should be directly differentated via graphical images in the listview. Pending notifications should be bold.
- 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.