[Open WebUI] Add more keyboard shortcuts, especially for prompt editing and Playground

**Is your feature request related to a problem? Please describe.** It's a bit subjective, but current selection of shortcuts is a poor fit. There's a shortcut for settings, which are rarely accessed and not keyboard-friendly anyway. On the other hand, I often edit the last prompt instead of asking followup questions and there's no shortcut to start editing it and no shortcut to submit changes. There are no shortcuts in the new Playground either, which makes it impossible to use the playground in the streamlined fashion described in #987. Notice the [wide range of shortcuts](https://docs.novelai.net/text/UserSettings/hotkeys.html) in NovelAI (although NovelAI made the mistake of using Alt, which is impractical on keyboards that replace right Alt with AltGr). Current WebUI shortcuts use three-key combinations, which are usually only used in text editors with several dozen shortcuts. Simpler two-key shortcuts should be used whenever possible. **Describe the solution you'd like**