Bring back script encryption in export preset

Retrieved working implementation from 2.1 branch and adapted to
existing export preset system.

Added Script tab in export preset to export script as raw text,
compiled, or encrypted (same as in 2.1). The script encryption key is
visually validated. The script export mode and the key is saved per
per preset in `export_presets.cfg`, so it makes sense to ignore this
file in version control system.

Each custom exporting procedure can retrieve an export preset set
during project exporting. Refactored project export dialog a bit to
allow easier code comprehension.

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