Task-Types: Do not show hidden task types to the user

Initial problem:

• Associated LanguageResources are removed after InstantTransale has pretranslated a file (TRANSLATE-2023). But as long as the pretranslataion is running, the hidden task will be listed for the LanguageResources that it is using.

Solution:

• Stop hidden tasks from being visible to the user.

Developer notes:

• Create an extra class for the task-type-handling. (Move the task-type-related code from the task-Model into this new class.)
• Implement a dynamic way of handling task-types and their visibility
• Keep in mind that in addition to tasks, we'll sooon have projects.
• Keep in mind that plug-ins can define task-types, too.
• see also: https://confluence.translate5.net/display/MI/Task+Typen