WIP: new feature "user preferences"

[TamaroWalter]

After completing the main functionality of townsquare (getting relevant notifications for the user) the plugin needs an enhanced user experience. The user should be able to filter the amount of events. To do that, this branchs adds a sidepanel with 3 kinds of filters:

• a course filter, where the courses that have notifications are listed. The users can (un)check courses to (not) see notifications from them
• a time filter, where different time spans are listed. The user can select different time spans.
• a "letter" filter, where the different letter categories are listed. The user can check them to see only specific kinds of notifications

Additionally, the user has the option to save the settings for future sessions. The setting are then saved in a database.