Added basic support for IconSource and its subclasses

[ShankarBUS]

Fixes #191

[ShankarBUS]

Should I also add the xml documentation comments?

[ShankarBUS]

@Kinnara, I would like to port the TeachingTip control from WinUI #88 once this PR is merged.

What do you say?

[ShankarBUS]

Hey @Kinnara,

I explored ModernWpf's source code.
I can see that you're using bindings instead of RegisterPropertyChangedCallback() which is not available natively in WPF. You have to add additional private dependency properties instead of just storing the registered token and revoking it.

This increases the inconsistency in code between WinUI and ModernWpf.

What I suggest you is to take a look at our toolkit and see how it handles this scenario.

I've made a DependencyObjectExtensions class which will do this for.

https://github.com/ModernWpf-Community/ModernWpfCommunityToolkit/blob/master/ModernWpf.Toolkit.UI/Extensions/DependencyObjectExtensions.cs

This will help us to reduce extra code and increase the consistency between the codebases.

My workaround is imperfect but with your help, we can improve it.

What do you say?

[ShankarBUS]

@Kinnara, all done! This PR is ready to be merged.

[Kinnara]

My workaround is imperfect but with your help, we can improve it.

What do you say?

The main road block is that we have to call RemoveValueChanged when using AddValueChanged, otherwise the DependencyObject will never be garbage collectied. When using WinRT's RegisterPropertyChangedCallback, there is no such issue.

[ShankarBUS]

We have to unregister the callbacks when unloading the control.

https://stackoverflow.com/a/45160026

This behaviour is same in WinRT, isn't it.

I've a little experience porting Controls from WCT so I can confirm they do the callbacks unregistering on unload.

If you say so, it must correct be correct 🤷‍♂️.

Thanks for merging this PR!