Remove pApplication::pendingEvents from the Windows and macOS hiro backends

[Screwtapello]

In commit 08e6de3, we removed pApplication::pendingEvents() since Qt6 doesn't support that functionality. Nothing else uses it but pApplication::processEvents(), so we should remove pendingEvents() from the other backends and fold it into processEvents() if that's appropriate.

b7678f5 made the same change for GTK.

We should do the same for Windows and macOS, when we can find somebody to actually test those platforms.