Variable DEMO_NODE_PREFIX`Const`

DEMO_NODE_PREFIX: {
    input: "omx-demo-in";
    main: "omx-demo-main";
    output: "omx-demo-out";
} = ...

The base name prefixes for the demo's virtual I/O nodes (hyphens keep them out of the mixer-owned omx_ patchbay filter, so they show as patchable sources / sinks).