[electron] Unhandled method validateKeyCode error in a bundled electron application

[kittaakos]

messages.js:46 Uncaught (in promise) Error: Unhandled method validateKeyCode
    at Proxy.<anonymous> (proxy-factory.ts:232)
    at KeyboardLayoutService../node_modules/@theia/core/lib/browser/keyboard/keyboard-layout-service.js.KeyboardLayoutService.validateKeyCode (keyboard-layout-service.ts:112)
    at KeybindingRegistry../node_modules/@theia/core/lib/browser/keybinding.js.KeybindingRegistry.run (keybinding.ts:614)
    at HTMLDocument.<anonymous> (frontend-application.ts:171)
ResponseError @ messages.js:46
handleResponse @ main.js:430
processMessageQueue @ main.js:258
(anonymous) @ main.js:242
runCallback @ timers.js:694
tryOnImmediate @ timers.js:665
processImmediate @ timers.js:647

I cannot share the effective package.json as is, once I managed to strip down the context, I'll update this PR with a reproducible configuration.

A few things before I start investigating the problem:

• I think we should not store the JSON files in the src folder: @theia/core/src/common/keyboard/layouts. For instance, for the TextMate grammars we use data, cannot we do the same?
• [minor]: As a convention, we use electron-node and not electron-main for the electron specific Theia backend bindings: @theia/core/src/electron-main/keyboard. See the Git Theia extension.

[spoenemann]

The problem is reproducible in @theia/example-electron.

[spoenemann]

I need to wait until #5088 is merged to avoid merge conflicts.