Shortcuts configuration

- [x] I have checked that a similar [feature request](https://github.com/Genymobile/scrcpy/issues?q=is%3Aopen+is%3Aissue+label%3A%22feature+request%22) does not already exist. **Is your feature request related to a problem? Please describe.** Some shortcuts are already used by other programs. For example, Alt+R and Alt+Z are used by NVIDIA's overlay functionality, and these shortcuts cannot be used in scrcpy. Although the MOD key can be modified, shortcuts with modified MOD key may still conflict with shortcuts from other programs. **Describe the solution you'd like** A configuration file for modifying all shortcuts **Describe alternatives you've considered** None **Additional context** None