Add option to return to default settings

[digitalethics]

Currently there is no option to globally reset all changes to the default settings. This would be very helpful to be able to experience Foliate the way it is shipped out-of-the-box (for testing and evaluation) and to give new users an option to reset the changes they've made after tinkering with it.

[johnfactotum]

You can reset all settings to default using gsettings or dconf editor.