Summary:
Zooming by mouse click/wheel and shortcut toggling from {nav Fit}/
{nav Fill} to {nav 100%} already centers the zoom to the current
cursor position.
This patch also centers the zoom on the cursor position for:
* triggering {nav Fill} zoom ({key Shift} middle click/{key Shift F})
* shortcut zooming in/out ({key Ctrl +}/{key Ctrl -})
* shortcut zooming to 100% (if `Actual Size` shortcut set)

Test Plan:
* Try different zoom actions by mouse/shortcut
* Check if zoom centers to current cursor position (inside image view)
* Check compare mode

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14181