Fix scrolling with touchpad when in Image view and mouse wheel behaviour set to Browse

Summary:
Similar to zooming using Ctrl+touchpad two-finger scroll, using touchpad scrolling
in image view when mouse wheel behaviour is set to Browse, browsing images is way
too fast.
This implements the same fix in {D7744} for this case.

BUG: 388353

Test Plan:
  # Settings > Image View > Mouse wheel behaviour = Browse
  # Open folder with a bunch of images, switch to Image view
  # Enable thumbnail bar
  # With mouse over image, scroll with the touchpad

Reviewers: #gwenview, ngraham, rkflx

Reviewed By: #gwenview, ngraham, rkflx

Subscribers: mart

Differential Revision: https://phabricator.kde.org/D11200