Use new error dialog in user retrieval in front-end
Add standard formatting header to user front-end file and reformat it
Make user profile handling more robust

This commit adds a new user profile function and prototype. When a user
profile object is created it checks if all expected fields are
available and throws an exception if not. The single place where a
user profile object is created can handle the exception and displays an
error dialog in such a case. Not being able to initialize the user
profile is a severe problem after all.

The reason for creating a user profile constructor and prototype is that
there has to be one place where information can be stored whether a
particular user profile option can be modified and made persistent by
a user. This is with respect to issue #590.
Support saving of specific user profile settings from front-end

This commit adds front-end and back-end code to allow regular users the
modifications of certain user profile settings. Currently, this is only
the display of reference lines in a stack. The back-end won't allow any
other setting to be modified through the interface added. To allow more
settings to be modified, they have to be added to the back-end code and
they have to be marked as modifiable in the front-end.

Anonymous users are never allowed to change the user profile of the
anonymous user. However, the back-also doesn't complain if the anonymous
user tries. This makes callback implementation in the front-end simpler.

This fixes issue #590.