Display non-public fields and properties

When debugging I typically need to see non-public information about an
element/object. For example for elements I want to see the _finalRect
(the last rect it was arranged with) or the _previousAvailableSize (the
last size it was measured with). Snoop only shows the members exposed as
property descriptors (e.g. public properties and attached dps) . So I
created some PropertyDescriptor implementations for 3 additional pieces
of information - FieldInfo, PropertyInfo (really just to handle the
non-public - i.e. internal, protected, private, internal-protected -
properties) and UncommonField<T>'s. The latter being how WPF manages
some private dependency property type information such as the automation
peer associated with an element or when implicit clipping occurs.