Improve filtering in project items list

[Nicklas Nordborg]

The ConditionalQueryElement class that was added in [7018] is useful for selective filtering. The code for listing project items should be modified so that it can use this kind of filtering.

After that we should add Subtype column to the list.

[Nicklas Nordborg]

(In [7032]) References #1975: Improve filtering in project items list

The list now uses the conditional query element. The filter implementation that enable/disable the restriction has been moved from the trashcan JSP file to a separate java class, ReturnClassFilter.

[Nicklas Nordborg]

(In [7034]) References #1975: Improve filtering in project items list

Added 'subtype' column to the project items list.

Also realised that when filtering on a column that isn't available for some item type, those items should in most cases not be included. The implementation of conditionalRestriction() now supports an option to exclude all item types not matching the filter. The trashcan list was updated to work like this.