Read and write PSXPrev.settings.json file

[trigger-segfault]

• Program settings are now loaded on startup and saved on exit (in the PSXPrev.settings.json file).
• Settings class now determines default values of all settings. Because of this, all checkboxes should start unchecked, and all numeric up/downs should start at 0 or their minimum value. To compliment this, any properties changed by these controls' changed events should also start in the false/0/min state so that changed events not firing (due to the value already being the same) will not cause inconsistencies with properties and UI controls. All checkbox controls must use the CheckedChanged event and not the Click event to handle the checked state being changed by settings.
• Newtonsoft.Json v11.0.2 was chosen, as its the last version that doesn't require a newer version of C# (due to nullable attributes).
• ExportModelsForm now properly updates enabled states if the default radio button is the one selected.
• ExportModelsForm and ScannerForm settings are now preserved (upon successful use of the form) when closing the PreviewForm.
• Moved ScanOptions out of the Program class.