Performance improvements for settings

[Wouter01]

Description

This PR addresses an issue where changing settings could be laggy.

The @AppSettings property wrapper has a new initializer, which accepts a keypath to a property in the SettingsData struct. With this, a view will only observe that specific part of the settings struct, resulting in less view updates when a setting has changed. This will hopefully fix the performance issues.

Related Issues

#1247

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Additional Context

Instead of writing this...

@AppSettings var settings

...you now need to get each setting separately.

@AppSettings(\.textEditing.defaultTabWidth) var defaultTabWidth

Or, if you want to observe a whole subpart of settings you can use...

 @AppSettings(\.textEditing) var textEditing

...which can ease the boilerplate a bit. However, the view will be updated on every change in the text editing settings, so it should be done with care.

The old initializer will display a warning...