Template download in background

[ariromano]

Godot version:
3.06

OS/device including version:
macOS 10.14.2

Issue description:
When downloading an export template through the editor, clicking anywhere else will not only close the window but terminate the download

Steps to reproduce:
fresh Godot installation.
fresh project
export, for example, for Mac
→ editor downloads 300+ MB but when clicking anywhere outside of the download window, it'll get interrupted.

→ download should run in background so user can continue working while downloading template.

[KoBeWi]

In 3.1 at least it doesn't close when you click outside.

[Calinou]

See also godotengine/godot-proposals#35.

This was implemented by #48902, closing.