When viewing an image in fullscreen mode and pressing {key F4} to
toggle the sidebar, the toolbar at the top does not resize.
The buttons on the right side are not visible while sidebar shows.
In view mode a custom widget is used, in browse mode a standard
`KToolBar` where this problem does not appear. Since the toolbar
overlaps the image view a standard layout is not usable here
which would send resize event automatically.
This patch adds a resize Eventlistener to adjust toolbar width.
BUG: 387784