`widget()` methods should be self-sufficient

[boonebgorges]

In order to work properly in the context of the Customizer, all widget() methods must be self-sufficient. That is, they should assume that the $instance array passed to the function may sometimes be empty. As such, each widget class should have a way of defining the default widget settings independent of the form() or instance() methods. A default_args() or default_settings() method seems like a reasonable technique.

See ​https://buddypress.trac.wordpress.org/ticket/6747#comment:5

[boonebgorges]

In 10384:

Make sure that max_groups is always defined in group widget.

This prevents a notice from being shown when the widget is added in the
Customizer.

Props slaFFik.
See #6754.
Fixes #6747.