Use indeterminate progressbars for editor downloads (export templates & assets)

[ckaiser]

trafficstars

Now that we have indeterminate progress bars, I've implemented their use for the most obvious case: downloads. There might be other places that could use them but it'd involve changes to ProgressDialog and tasks, haven't looked through that as of yet.

Before & After:

https://github.com/godotengine/godot/assets/138269/7643d5d1-dd94-441d-8a95-30d5211db6fb

https://github.com/godotengine/godot/assets/138269/1184a474-176f-4311-b7ab-0de7bd8de632

---

https://github.com/godotengine/godot/assets/138269/5eab6a2d-7eef-43d3-8613-63170d2ea826

https://github.com/godotengine/godot/assets/138269/ddec0abd-18a9-4813-a286-e7f791e2d575

(I was connected to a slow VPN for testing I swear I'm not on dial-up or something 😅)

[ckaiser]

Fixed the redraw issue

[akien-mga]

Thanks!