Fix infinite progress bar on import animation frames

Before this patch, when you tried to import one picture
using Import Animation Frames feature, there was an infinite
progress bar that was not possible to get rid of.
This commit fixes that behaviour (progress bar fills itself
and disappear when the image is loaded).

BUG:402625

Tests:

• import one .png file
• import one .gif file
• import one .kra file
• import multiple .png files