Default hotkey conflict with Windows Magnifier

[steinbro]

The default rust-reader hotkey to decrease speaking rate, Ctrl+Alt+minus, is the same as the Windows Magnifier hotkey for toggling 1x magnification, per https://support.microsoft.com/en-us/windows/magnifier-keyboard-shortcuts-and-touch-gestures-6d7c3095-75ec-258f-6f5b-3b0bc19a18e7

rust-reader crashes on startup if a hotkey fails to bind. A hotkey already bound to another application will cause the binding to fail. This leads to the quirky behavior that, under the default configuration, rust-reader and Windows Magnifier can run concurrently, but only if rust-reader is launched first.

I propose:

1. Change the default rate-down hotkey. Screen reader + magnifier are a typical combination of accessibility tools, so ideally rust-reader wouldn't have incompatibilities with the built-in magnifier. I'm not aware of a way to change the Ctrl+Alt+minus hotkey in Windows Magnifier.
2. Provide better user feedback when rust-reader fails to launch due to a hotkey conflict. Right now, it just crashes when trying to unwrap a None value returned by HotKey::new().