Fixed Default Button Behavior

If Default Button is not visible or not enabled, pressing the enter or
space keys does not trigger button press action.
Fixed deletefcn

If object is invalid at the time of attached HG deletion, take no
action.
Bug fix

Incorrect method tied to elements' ObjectBeingDestroyed listener
Improved resizing behavior with spanned elements
Added 2 new properties

HorizontalWeight and VerticalWeight to mimic the behavior of
uigridcontainer, specifying the space distribution among grid
columns/rows.
Added object validity check
Fixed traversing panel children setting Enable
Edited inline help text
Updated set.DefaultValue behavior

If the property value has not changed from its default value, updating
DefaultValue also updates the property value