Fix Label Value add
Problem description:
Currently when ColumnDock::addLabel() is called, the mode of the labelsMode() is used. This variable is not initialize and therefore the following switch will not handle the case in the most cases, because the mode is random. Therefore it is not possible to add manually value labels
- give variable an initial value. This value does not have a meaning if valueLablesInitialized() returns false, but at least it returns a valid mode
- Initialize value labels with column mode if not yet initialized
- rename function to a better name to have initialize, deinitialize and valueLabelsInitialized for checking