multi-employer configuration and time-tracking

[mathisdt]

TWT should support an arbitrary number of employers to be configured.

• Each employer can have different settings (so the options have to be employer-bound and can't stay standard Android properties).
• Each employer can have different tasks.
• The employer should be selectable on top of the main screen. Everything below should change accordingly. The app should remember the last selected employer. If only one (the standard) employer is configured, the employer selection should be hidden.
• The events and target times the user enters count for the currently selected employer.
• The notification and the widget should both display the time/data for the employer which was last selected in the app.

This probably will be a big change, especially in the data model.