SDC component prop widgets - validations, improve settingsSummary(), etc.

Problem/Motivation

The prop widget settings forms can be fragile in some cases particularly the more complex array & object structures. Now that we have refactored the MultiValue form element to allow item deletions and other improvements, we can audit the widgets more carefully and harden them up.

Proposed resolution

• The PropWidgetArrayObject widget needs validation to prevent values with all empty properties from gettings saved into config.
• The settingsSummary() needs to be a little more systematic so that parent widgets can pass down indentations and such
• The PropWidgetBase has a default 'widget' of #type => 'value'. We need to change this to #type => 'hidden' so it gets passed through validation functions.
• Add utility helper functions & constants to SdcTrait to help with validations such as stripping out irrelevant keys, determining empty, etc..
• Evaluate any known missing prop widget support and/or settings