* pkg/plus/dpp is the main plugin bridge, and defines nothing but an interface
that defines the Doodle++ surface area (referring to internal game types such
as doodad.Doodad or level.Level), but not their implementations.
* dpp.Driver (an interface) is the main API that other parts of the game will
call, for example "dpp.Driver.IsLevelSigned()"
* plus_dpp.go and plus_foss.go provide the dpp.Driver implementation for their
build; with plus_dpp.go generally forwarding function calls directly to the
proprietary dpp package and plus_foss.go generally returning false/errors.
* The bootstrap package simply assigns the above stub function to dpp.Driver
* pkg/plus/bootstrap is a package directly imported by main (in the doodle and
doodad programs) and it works around circular dependency issues: this package
simply assigns dpp.Driver to the DPP or FOSS version.
Miscellaneous fixes:
* File->Open in the editor and PlayScene will use the new Open Level window
instead of loading the legacy GotoLoadMenu scene.
* Deprecated legacy scenes: d.GotoLoadMenu() and d.GotoPlayMenu().
* The doodle-admin program depends on the private dpp package, so can not be
compiled in FOSS mode.
