Convert the IsOpenChanged event to a RoutedEvent

Fixes #1437
Move the CustomValidationPopup class into its own file
Apply the AttachedPropertyBrowsableForType attribute only to the getter

Should fix a bug similiar to 7a94bede2dbe150249969d34fed2643d845f34bf
Apply a negative margin to the CheckBox, so it behaves like the RaidoButton and the space between the box and its content can be clicked to change the CheckBox's state
Create 1.0.0.md
Merge remote-tracking branch 'upstream/master'