Need to determine how default property values should behave

[zastrowm]

We're inconsistent on how default property values should behavior. For instance, BackgroundColorProperty returns '' when no value is set, but may have a default value set by the theme. On the other hand, HorizontalAlignmentProperty returns the computed value by default (center, but is settable via theme) and doesn't really support returning "I'm using the default value"; FontSizeProperty behaves sort of in the same way as HorizontalAlignmentProperty.

We need to determine if we want computed values to be returned by getValue. Perhaps we need both getAppliedValue and getCurrentValue; where the former is explicitly set values, and the latter is the the value, even if it's computed from the current theme.

We should also consider if this is only a problem for "layout" properties, because of theming, or if it's a generalized issue.