Filters can be fixed and applied with default values

This FR builds on the vision laid out in Category for Huge Amount of Data. 
This project depends on / requires that the following list of PR and FR is accepted and implemented 

• Pull Request UI Filters (Implementation)
• Pull Request UI Filters
• Filter-Configuration in Categories
• Issue: Support Filters in Manage-Tab of Categories

1 Initial Problem

Some Organisations want to offer a subset of their courses to users. They want to use pre-set filters to  ensure no further user interaction is required. 

2 Conceptual Summary

As a repository alternative to Course Catalogue it is suggested to be able to offer categories that offer a specific selection of objects (possibly to specific roles) by pre-set filters. 

1. Filters in category are activated. Filters are set to be fixed and hidden. 
2. Filter fields are selected and saved. 
3. Filter values are entered or selected. 
4. Categories shows only objects complying with the pre-set filter values. 

3 User Interface Modifications

3.1 List of Affected Views

Category > Settings
Category > Content 

3.2 User Interface Details

3.3 New User Interface Concepts

None.

4 Technical Information

{The maintainer has to provide necessary technical information, e.g. dependencies on other ILIAS components, necessary modifications in general services/architecture, potential security or performance issues.}

5 Contact

• Author of the Request: Tödt, Alexandra [atoedt]
• Maintainer: {Please add your name before applying for an initial workshop or a Jour Fixe meeting.}
• Implementation of the feature is done by: {The maintainer must add the name of the implementing developer.}

6 Funding

If you are interest in funding this feature, please add your name and institution to this list.

• …

7 Discussion

Killing, Alexander [alex], 16 May 2019: I support the idea in general. The screen workflow is a little bit unclear to me.

Kunkel, Matthias [mkunkel], 16 MAY 2019 : How do one get on screenshot 1 and 3 ? Screenshot 2 can be combined with request Mass Category filtering lists objects from sub-tree. But how we combine 1 and 3 with other filter requests?

Klees, Richard [rklees], 20 MAY 2019: How will this look to the user? Will she be aware, that the content of the category is filtered?

Tödt, Alexandra [atoedt] The organisation requesting this feature does not want to inform users. 

JourFixe, ILIAS [jourfixe], 20 MAY 2019: We highly appreciate this feature request and accept it for 6.0 with the following changes:

1. We refuse the current suggestion of hiding the filter and prefer to only grey-out it. This would make it more clear what is presented to users on the screen (and probably reduce user support).
2. Instead of one setting "Fix filter values and hide filter" we would like to have one setting for applying default filter values and one for greying-out the filter to prevent changes to filter.

8 Implementation

{The maintainer has to give a description of the final implementation and add screenshots if possible.}

Test Cases

Test cases completed at {date} by {user}

• {Test case number linked to Testrail} : {test case title}

Approval

Approved at {date} by {user}.