Add color setting
This introduces a
ColorSetting, which may be used by SettingsDialog or other settings.
I added the new
ColorSetting to the account management dialog, so users are able to choose colors for their accounts (which may later be used elsewhere). Initially, the color value is generated by using the account's JID.
What still needs to be done:
- config migration fails because text_to_color fails while migrating settings (app.css_parser.prefer_dark is not available at that point)