Add simple "reset to defaults" button of all Aseprite settings

[Gaspar Capello]

UI elements "reset to factory" requested here (internal ticket): https://igarastudio.zendesk.com/agent/tickets/6089

This can be fixed by using a simple global button to reset all settings (same effect as deleting preferences folders).

Or a better idea (in my opinion): have a centralized "Reset" item in Preferences that contains all Aseprite "reset to default" sectors.