Handle opening 2 instances of GDevelop

[ClementPasteau]

• this creates a window inside the existing electron app, instead of a new app, allowing to have everything under 1 app in the taskbar
• it is created in a different context, allowing indexedDb and storage to be different, for firebase to be independent, so allow logging in to different accounts
• previews are independent, so we can start previews for each window and they're fully working
• Fix closing a window now automatically closes the preview(s) associated to avoid having a crash when closing the preview
• protect all calls to avoid sending messages from/to non-existing sources