Update to Electron 10

[starpit]

This will require test updates, due to Spectron's switch to webdriverio v6. The webdriverio API changed substantially. For example this.app.client.getText(selector) must now be written as this.app.client.$(selector).then(_ => _.getText()).

Also, electron 10 will emit deprecation warnings, due to our need to disable allowRendererProcessReuse. electron/electron#18397

(electron) 'allowRendererProcessReuse' is deprecated and will be removed.

This is due to node-pty not yet supporting context aware modules: microsoft/node-pty#405