Avalonia settings dialog

[daniel-lerch]

Context

The current settings dialog especially on the tab Practice settings has a poor readability. Custom special char keyboards are hidden in another form despite being local settings as well.

Proposal

• Implement a modern settings dialog with Avalonia
• Use more screen space for better presentation
• Move settings for custom special char keyboards there
• Migrate special chars to the new settings file
• Leave old special char text files for older versions