Unify Minus/Subtract virtual keycodes

On all platforms other than Linux/X11, the Subtract key was uniformly
used only for the Numpad. To make this cross-platform compatible, the
`-` key will now map to `Minus` on X11 instead of `Subtract`.

Since people have been confused about the difference between `Minus` and
`Subtract` in the past, the `Subtract` key has also been renamed to
`NumpadSubtract`. This is a breaking change that might be annoying to
downstream since there's no direct improvement, but it should help new
users in the future. Alternatively this could just be documented, rather
than explicitly mentioning the Numpad in the name.

src/platform_impl/web/web_sys/event.rs

@@ -204,7 +204,7 @@ pub fn virtual_key_code(event: &KeyboardEvent) -> Option<VirtualKeyCode> {
         "Slash" => VirtualKeyCode::Slash,
         "Sleep" => VirtualKeyCode::Sleep,
         "Stop" => VirtualKeyCode::Stop,
-        "NumpadSubtract" => VirtualKeyCode::Subtract,
+        "NumpadSubtract" => VirtualKeyCode::NumpadSubtract,
         "Sysrq" => VirtualKeyCode::Sysrq,
         "Tab" => VirtualKeyCode::Tab,
         "Underline" => VirtualKeyCode::Underline,