[FEATURE] Keep overlays enabled if no EXTCONF exists

Since these checkboxes are new the overlay functionality would be disabled when when a user updates to 1.3 because there is no EXTCONF.

Checking for isset enables the feature if a user comes from earlier versions.

For fresh installation of EXT:view the default values are written to EXTCONF automatically so checking for ISSET is a good way to keep the feature enabled for update but have it disabled by default for fresh installations.