inject: queue an input as if typed

[O6lvl4]

For a native menu bar whose items do what their key equivalents do. A menu item fires inside the event loop, and the key it takes never reaches the window. The program's own native code can call crate::window::inject(kind, code, mods) to hand the program the same input instead.

The inputs go to a thread-local queue apart from the host, because the host is borrowed while the loop runs. They join the window with the focus (else the current one) after each pump, and the loop is woken.

Checked: an injected TEXT 'c' with Super arrives after the window's FOCUS event, as kind 5, code 99, mods 8, window 1.