SettingsController: Add reset feature + small improvements

This merge request brings an addition to the settings by:

• Adding a new section in the Settings tab introducing a feature to allow users to reset all the settings to their default values.

This merge request also brings a fix to:

• The setting specifier that is now checked before being used in order to avoid crashing the application by accessing its variables in the case where the specifier does not exist.
• Some default values that were not properly declared or had the wrong type.

Closes #1761 (closed)

Screenshots