Feat: Resetting values in SettingsBase

[yoshiask]

Background

This PR introduces two options for resetting a setting to its default value. Closes #3

Motivation

Resetting could previously be achieved by setting the property to null, but this did not fire PropertyChanged and could be unintuitive for consumers.

Proposal

• Fix #3 by making SetSetting<T>(T value, string key) always invoke the PropertyChanged event
• Add ResetSetting<T>(string key) to allow explicit resetting of properties

[yoshiask]

@Arlodotexe Would it also help to add a method to reset all settings (clear _runtimeStorage)? Maybe that should be a separate PR, but it's still under the umbrella of "resetting values"

[Arlodotexe]

@Arlodotexe Would it also help to add a method to reset all settings (clear _runtimeStorage)? Maybe that should be a separate PR, but it's still under the umbrella of "resetting values"

@yoshiask Sure, we can make that part of this PR if you'd like, otherwise this looks good to me. For naming, I'm thinking something like ResetAllSettings()?

[yoshiask]

Yup, that name sounds good to me