Improve GUI for selecting parent/child item columns

[Nicklas Nordborg]

When selecting new columns from parent/child items, the list of columns can be quite long. It would be nice to replace this field with something that looks nicer and where it is easier to find the correct column. For example, it might be a good idea to clearly separate static columns, from annotations and extension columns.

[Nicklas Nordborg]

In 8070:

Fixes #2283: Improve GUI for selecting parent/child item columns

The list with columns has been replaced with a table that has 3 columns:

• Static properties
• Annotations
• Extensions

Columns are selected with checkboxes.

[Nicklas Nordborg]

In 8074:

Fixes #2283: Improve GUI for selecting parent/child item columns

Added possibility to select select a column from a related item with two hops. The first hop must be to a parent item and the second to a child item.

[Nicklas Nordborg]

In 8075:

Fixes #2283: Improve GUI for selecting parent/child item columns

Updated the documentation.