webview_exit crash on win7 mshtml

Env: win7 64, toolchain: x86_64-pc-windows-msvc, try example `dialog` and click `Exit` will get crash, but click the close button on title is ok. The below code patch seems work:
```
WEBVIEW_API void webview_exit(webview_t w) {
struct mshtml_webview* wv = (struct mshtml_webview*)w;
/* DestroyWindow(wv->hwnd); */
PostMessage(wv->hwnd, WM_DESTROY, 0, 0);
/* OleUninitialize(); */
}
```

Start by reproducing the crash with the `dialog` example on Windows 7 x86_64-pc-windows-msvc, then inspect the `webview_exit` path for the `mshtml_webview` implementation. Compare the suggested `PostMessage(WM_DESTROY)` behavior with `DestroyWindow` and `OleUninitialize`; done means clicking `Exit` no longer crashes while the title-bar close still works.

Written by the indexing model from the issue text.