Windows: Delayed Message Boxes Fix.

[VZout]

Message boxes did not appear when opening them. The sound did play but they never were made visible unless the system triggers a redraw (for example when you press alt). The solution to this is to add the default WM_PAINT behavior back in. (DefWindowProc does this if you don't override WM_PAINT)

(This issue appeard in #1615)

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[maroider]

I ran ahead and tried your changes and it seems this fixes the problem on my machine (latest Win10).
This also seems to fix another problem I observed with message boxes and filed dialogs parented to the Winit window where instead of flashing, the child window would toggle the color of its title bar every time I clicked the Winit window.

[VZout]

CI now fails on docs after I updated the changelog...

Seems to be unrelated though: failed to load source for dependency bitflags

[msiglreith]

thanks for looking into this!