Reduce hyper-sensitive touchpad scroll-zoom speed

Summary:
BUG: 378584

Reduce hyper-sensitive touchpad scroll-zoom speed in Gwenview

Test Plan: Submitting this on behalf of Thomas Mitterfellner since he's having trouble with his Phabricator access. Patch is modeled after Albert's similar patch for Okular in https://cgit.kde.org/okular.git/commit/?id=7a50ce0edfc9be8bd23441e52a4f3a0c60f7e60f. Thomas has tested locally with a touchpad and scroll wheel, and I have tested in a KDE Neon VM. Seems to work as expected.

Reviewers: KDE Applications, broulik, aacid

Reviewed By: KDE Applications, aacid

Differential Revision: https://phabricator.kde.org/D7744