WM_DESTROY callback must return 0 (fixes TypeError from show_toast())

[f3ndot]

fixes #112

The on_destroy method that's called as part of the WM_DESTROY callback must return 0, not none. Otherwise this error will raise when using show_toast():

TypeError: WPARAM is simple, so must be an int object (got NoneType)

[xboxone08]

It's a fix for a minor annoyance, so merging good 👍.

[fspofficial]

I have made much more improved changes by just changing a single line- #118

[xboxone08]

I have made much more improved changes by just changing a single line- #118

Uhh, yours looks the exact same?

[dmtzs]

Working correctly