AppBar: hide home button in course list
Assign; hide date block when the dates are off
Bugfix: Ext.Date.format returned wrong dates
Connection check added

Check the connection availability before any request.
Perform the request only if the connection is available.
Removed the modules count in courselist
Droped the store filtering with filterBy()
Update database per course, when course selected

- Update the database per course and do it when
  the course is selected. The update is done only
  once, during the first selection. Afterwards the
  local data are used.

- Use icons for the update statuses.

- Bugfixes: fields in the formpanel cannot be named 'id'
  Doing so messes the navigation history. The assignment
  were affected by this.

- Dropped the use of the filters for the stores. This only
  led to trouble. Using filters caused to have double entries.
  The new mechanism is the creation of smaller stores with
  selected content.

- Switched to filefield xtype instead of the textfield xtype
  with the file component.