Add UI for configuring input.conf

[anhtumai]

Overview Description: Now, to configure shortcut or application settings (exp: backward, forward, playback speed step size, ... ), users need to prepare input.conf file somewhere and upload it to Celluloid setting. This process is not obvious initially and requires users to do some investigation beforehand

Expected Results:
Would it make sense that we make UI for configuring input.conf like how VSCode does:

Under the hood, it will compile to input.conf and feed that to the application

Version: 0.27

Additional Information: This application is perfect, very customizable and has prettiest UI in GNOME. Thank you for creating and maintain such a beautiful application

[anhtumai]

a UI workflow is needed for this ticket. Any UI workflow suggestion is welcome.