Update titlebar after image modification

Summary:
In some cases the titlebar information (filename, size, zoom) is lost after an image has been modified.
This patch fixes the caption update and thereby also the missing `*` on modified images.

BUG: 345980

Test Plan:
Case 1:
1. Open image in view mode, which fits in window (100% zoom)
2. Scale (Shift-R) or crop (Shift-C) image
-> Titlebar info is lost

Case 2:
1. Open big image in view mode, which is zoomed to fit in window
2. Scale image with Shift-R (do not save)
3. Go to another big image (zoomed to fit)
3. Scale image with Shift-R
-> Titlebar info is lost

Reviewers: rkflx

Reviewed By: rkflx

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D9293