Provide a tag picker for macro parameters

Some macros take a list of tags, and the Edit Macro dialog shows it as a plain text field: the user has to type each tag exactly, case included, and a tag that no page carries is accepted silently. The tags of a page are already entered with suggestions of the existing tags, but no displayer offers them to a macro parameter.

Where it is needed

Proposal

• Add a Tag type, with its converter, so that the macro editor can tell a list of tags apart from a plain list of strings.
• Add a #tagPicker Velocity macro, which suggests the existing tags and takes several values, and an HTML displayer edit template for List<Tag> that uses it.
• Declare List<Tag> as the type of the parameter above, so that the Edit Macro dialog shows the picker, and turn the typed value back into the comma-separated list that the macro expects.

An implementation is proposed in PR #5999 of XWIKI-23834, for the notifications macro. The activity macro is left out: it is a legacy macro, to be deprecated in XWIKI-25515.

Impact found on macro fluidity

This KO comes from the WYSIWYG usability audit of the macros, under the criterion C4 input-assistance of its usability criteria. The page of the macro, linked in the table, has the full notes.