GameScope: Add default button

[sonic2kk]

A somewhat prerequisite for #838, because it will make the transition easier for users.

Adds a Default button to the GameScope button bar, so users can reset all options. This will be useful for #838 because it will introduce a breaking change for all GameScope GUI elements, since the order of elements will change entirely with several being merged into one.

This also fixes a bug that should never occur but that may occur in certain circumstances. When a dropdown value is blank, that option would be written out to the options because we only check if the value is not the default and not if it is not defined at all. So if GSFOOR was "", that is != to the default, so we wrote out --force-orentation with an empty value. This should never really occur but it happened during testing, so I fixed it when a -n check for GameScope dropdown variables.

[sonic2kk]

Shellcheck is good on this, this is probably good to merge.

[sonic2kk]

This option does not impact the gamescope checkbox itself, as this is stored in a different variable. Imo it makes more sense to keep this as whatever it was before the defaulting.