Fix SelectedItemsChanged event arguments in SelectingItemsControl

[WojciechKrysiak]

• What does the pull request do?
  Corrects the event arguments for replacing a selected item in SelectingItemsControl.
• What is the current behavior?
  The arguments for the SelectedItemsChanged event are inverted.
• What is the updated/expected behavior with this PR?
  Sets the arguments as expected.

Checklist:

• Added unit tests

[jmacato]

Idk if this should be added to the breaking changes. Someone might've been relying on the wrong parameters.

[WojciechKrysiak]

I haven't found any issues referencing this bug, so it's unlikely anyone quietly assumed this will remain broken forever. I'd say it's more likely no one used this feature until now.

[danwalmsley]

Wow thats a good spot! @jmacato should we get this into 0.7?

[jmacato]

@danwalmsley I do think this is a critical bug fix that needs to be included with the release
@WojciechKrysiak hmm better be safe and let's add it there

[WojciechKrysiak]

@jmacato done: https://github.com/AvaloniaUI/Avalonia/wiki/Breaking-Changes

[jmacato]

@WojciechKrysiak Thanks!