Add visibility and visibilityOptions parameters to the messageSender macro

Currently, the messageSender macro decides for itself, based on the page it is used on, what is the default selected visibility option.

The choice is rather simple:

• if it's a group page, visibility: group, value: current group
• if it's a user page
  • if the current user is the same as the user in the page, visibility:
    followers
  • if the current user is not the same, visibility: user, value: current user
• otherwise, visibility: everyone

Also, the list of available visibility options is taken from the XWiki.MessageStreamConfig class, as the possible values of the 'visibilityLevel' property.

This issue proposes to allow a macro caller to override this mechanism and specify what is the currently selected (default) visibility level and what are the available options to choose from.

The idea is to add 3 parameters:

• visibility: default selected visibility
• visibilityParameter: in case of a visibility level that accepts a parameter (user/group)
• visibilityOptions: comma separated sublist of the default ones