HandleWindowProc implementation conflicts with other plugins on Windows

[jmatth]

Describe the bug

In app_link's HandleWindowProc method on Windows, there is a single branch that can return a non-null result, and therefore prevent other plugins from getting the chance to handle the message. In my particular case this means that contextual_menu fails to trigger any callbacks. Presumably other Windows plugins experience similar bugs if app_links's handler happens to be called before theirs. Commenting out the default branch of that switch case resolved the issue for me without breaking my deep link usage, but I assume it's there for a reason and removing it would break other supported use cases.

Does it related to

[ ] App Links (Android)
[ ] Deep Links (Android)
[ ] Universal Links (iOS)
[ ] or Custom URL schemes? (iOS)

Does the example project work?

[ ] Yes
[ ] No
[x] Irrelevant here

Did you fully read the instructions for the targeted platform before submitting this issue?

Uploaded your files to webserver, HTTPS, direct connection, scheme pattern setup, ...

[ ] Yes
[ ] No
[x] Irrelevant here