[3.x] Improve the UI/UX of the Export Template Manager dialog

[YuriSizov]

Backport of #48902. After this is merged, #35868 can be tested again.

There is some small glitch when you close the window while downloading and reopen it. For a split second an error shows up, but is quickly replaced by the status update. Not sure what's up with that, but it's probably not directly related to this PR. Either way, it's not really disruptive and goes away momentarily.

[AaronRecord]

When I run it I get the following popup error "Error getting the list of mirrors.". I'm guessing this is just because it's not an official build? It makes it annoying to test though, as it won't let me "Download and Install". Is there a workaround?

[Calinou]

Is there a workaround?

You can edit version.py to make your Godot build pretend it's from another version, then recompile the editor.

That said, we should probably disable the download button and set a tooltip when using a development build (dev status) or non-numbered alpha/beta/RC build. There will never be official export templates available for those, as official export templates are only available for stable releases and numbered alpha/beta/RC builds.

[YuriSizov]

@Calinou The button is disabled and there is such tooltip already 👀

[akien-mga]

Thanks!