[electron example] Add plugin support

[ciampolo]

Description

So I have placed the vscode chrome debug extension inside the theia git root folder/plugins. When I run the theia electron variant there isn't any plugin view at all and the extension isn't registered (electron theia says there is no "Chrome" debug type). I checked the stdout of the yarn run start command and the logs of the devtools, nothing about plugins or the chrome debugger. This also happens without any plugins at all. When I use the Browser variant everything works flawlessly.

OS: Linux
Theia: Latest upstream and 0.5.0

Am I missing something obvious or is this just a bug?

[marcdumais-work]

The plugin extensions are not currently part of the Electron example application. I do not know if it was just missed or if it's because there's an issue.

Untested patch where they are added here

[ciampolo]

While the plugin extension is loaded correctly now it doesn't actually load the vscode extensions.

root ERROR Cannot find a service for the path: /services/plugin-ext
root ERROR Cannot find a service for the path: /services/plugin-paths

Your reverts #3737 so it should work but apparently there is other code left to modify.

[benoitf]

FYI it was disabled by #3723

[akosyakov]

Someone should fix #3651 and #2210 then we can enable it.