taskmanager: Add fade-in animation after the thumbnail image is ready

Fushan Wenrequested to merge
work/fusionfuture/pipewire-animation into master

On Wayland, the task thumbnail takes some time to load. After it's loaded, a fade-in animation can make the visual experience much better.

BUG: 449189 FIXED-IN: 5.25

Depends on plasma-workspace!1405 (merged)

Before After
2022-01-26_18-50-01 2022-01-26_18-46-28
Edited by Fushan Wen

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