TrackBar_Mixin : MixinThe following procedures are 'Message Notifications'; that is to say,
Windows has committed the event, and is politely notifying us.
The notification messages are 'stubbed' (cancelled), as you can
safely ignore them. Remember the Current_Position property is always
valid, so you can augment that property being SET, if you want constant
updating. alternatively, OnFinishedSliding is always sent after each
update. If you only wanted to be notified at the end of any sliding
operation (which includes keyboard support) then OnFinishedSliding is
all you need to trap.