Preventing deadlock on AppDomain.Unload by removing superfluous finalization logic which uses Dispatcher.

[BrunoJuchli]

The FlipView and the MetroTabItem contain finalizers which don't clean-up unmanaged resources. All they do is deregister a local handler from a local event, which is quite superfluous. What's more, is that the deregistration was done through the dispatcher. This can lead to a deadlock when unloading an AppDomain (btw., the deadlock is resolved. After a while AppDomain.Unload(..) throws an exception stating that it can't unload the domain).

The FlipView finalizer was originally introduced by commit b5aac16.
As far as i can tell the finalizer is not related to the issue which was handled by said commit Fixed a bug where the FlipView was unloaded by the TabControl but it`s inner events weren't reattached when the Tabitem was switched back

[BrunoJuchli]

I've also looked at all other finalizers in the project. They all clean up unamanged resources except for NumericUpDown.cs. There the finalizer is doing

DataObject.RemovePastingHandler(_valueTextBox, OnValueTextBoxPaste);

(Note: The Pasting handler is attached in the OnApplyTemplate method)
Couldn't that be moved to the/an Unloaded event handler instead of the finalizer?
The finalizer is for freeing unmanaged resources.

[flagbug]

Yeah, you're probably right, no idea why there's stuff in the finalizers

[BrunoJuchli]

To be honest i don't know WPF very well. I'm a bit uncomfortable with manually regression testing NumericUpDown and that's why I've refrained from altering it, too. Shall i create an issue for that, so that someone can look into whether RemovePastingHandler should go into an Unloaded event handler or some other place? I could just do it and create a pull request, but no warranties that it's still working, then ;-)

[flagbug]

Yeah, can you create an issue so that we can keep track of the NumericUpDown thing?
You're also certainly welcome to create a pull request for reviewing, we don't have to merge it 😄

I'll merge this PR right now