Add support for the OS task bar progress API

[havi05]

Describe the project you are working on

Program updater with a loading bar.

Describe the problem or limitation you are having in your project

I can't show the progress in the TaskBar.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add a OS methode (Like OS.request_attention()) with a percent value.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

OS.taskbar_value = PERCENT (Like $ProgressBar.value = 60)

If this enhancement will not be used often, can it be worked around with a few lines of script?

No.

Is there a reason why this should be core and not an add-on in the asset library?

It is an addition to the OS..request_attention() methode.

[Calinou]

See also #477 and godotengine/godot#5844. There's a bit of discussion about implementing this feature here: godotengine/godot#5560 (comment)

Note that Windows' taskbar progress bar supports defining three states as well:

• Active (displays in green)
• Paused (displays in yellow)
• Error (displays in red)

I think there's also an indeterminate state (when the percentage isn't known), but I'm not sure.

[havi05]

Yes, there is an indeterminate state. (pulsating green or sliding green ; I don't exactly know)