Allow entering format strings that merges data from multiple columns into one in the User Directory

In XE 2.5 the User Directory displayed a 'Members' column with the format 'Ecaterina Moraru (XWiki.evalica)' (see XE-432)

In XE 4.0 (see XE-1131) the columns have been individualized (for a better sorting and filtering) into individual columns 'User ID', 'First Name', 'Last Name'.

XE 4.3 added an user displayer (see XWIKI-4022 ) that grouped the Picture, First Name, Last Name and User ID into a single displayers, that can be associated to a single column.

From a consistency point of view we need to display the users in the same way across the application (livetables, administration, send message, etc.).

User Directory columns are ment to be used for filtering and sorting of user information, so we should allow users to have separate columns with the ID, last name and first name, but by default, we should display the users columns merged in a consistent manner (see livetableDisplayer.png and XWIKI-7897)

Feature discussed with Eduard Moraru