Nextmatch widgets
Nextmatch (api/js/etemplate/Et2Nextmatch/) is the searchable/sortable/filterable list widget
built on top of Et2Datagrid. Its header family - the base
Nextmatch Header and its variations (sortable headers, and
the various filter headers that are really Et2Select/Et2SelectAccount/Et2LinkEntry
subclasses repurposed as column filters via FilterMixin) - all live in the
Data Grid sidebar category alongside Nextmatch itself, since their whole purpose is
grid-specific even where their actual implementation inherits from an unrelated widget elsewhere.