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

[ClementPasteau]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Propagate second-instance args to new windows

When a second instance is launched, you parse secondInstanceArgs and create a new window, but the renderer still reads arguments via remote.getGlobal('args') (see Window.getArguments()), and global['args'] is only set once at app startup. That means windows opened from a second launch will still see the first instance’s arguments (e.g., the project path or --disable-update-check), so the new window won’t open the intended project/flags. This shows up when users launch the app a second time with a different project path.

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[ClementPasteau]

closing for #8205