Ensure options are met when setting or getting settings

Closes #748 (closed)

Up until now we did not check whether the new value we set is in the options of a SettingEntry.
Now we check this while setting to prevent wrong data and when getting to ensure we get a valid value even if we changed something with the settings (e.g. changing the type or removing an option).

This is testable by setting e.g. calling this.settingsService.set('transmission-send', {bla: 'Qerw'}) somewhere where the settingsService exists (e.g. settings overview page) before checking out this branch.
This will result in the send option in the settings to be blank. After checking out the new branch we should see the defaultValue selected.