Add missing NotifyIcon notification constant

[riverar]

Fixes: #1765

[mikebattista]

What's blocking it from being int?

[riverar]

@mikebattista I didn't fully explore that rabbit hole, but I think we'll need to change WM_USER and all the NIN_ msgs to int and then fix all the collateral damage. 😅

[riverar]

Interesting, made the change locally again and doesn't seem as bad. Will update this PR shortly.

[riverar]

Looks like AzDO has given up on builds. @mikebattista

[mikebattista]

All the other WM_ constants are uint. Are there side effects of making WM_USER int but not the others? Feels like this should cause some usability problems somewhere.

[riverar]

Yeah, I think there's a larger "naked constants should default to int not uint" discussion to be had here.

[mikebattista]

Looks like AzDO has given up on builds. @mikebattista

Thanks. I believe it should be fixed now.