Internal: Save new main settings fields to file already during loading

[Taitava]

Currently, if a new version creates new settings fields, they are only saved to disk when a user changes settings (= causes a write).

• Exception 1: If there are new shell command fields, saving is done at startup.
• Exception 2: If there are changes that require migrating old settings data, saving is done at startup.

In other words, this change will concern quite small number of situations, but I want the behavior to be consistent. A real practical difference is that if new field creation does not happen at startup, no backup file will be created from the settings. I want it so that even a small change to the structure of the settings file will trigger the backup file creation.

[Taitava]

Done.

[Taitava]

Released.