Offer a way to clear hotkeys

**Is your feature request related to a problem? Please describe.**
It's possible to move hotkeys, but not clear them

**Describe the solution you'd like**
LiveSplit uses "Escape" to clear, it's also a common key to do so
We could also use "right-click" to clear.

**Describe alternatives you've considered**
It's currently only possible through editing the settings.toml file
