Gwenview allows to Ctrl-click to zoom in, and
Ctrl-right-click zooms out. However, when clicking fast only every
other click resulted in a zooming action, making the experience feel
slow and cumbersome.
The cause for this is that fast clicks are detected as double-clicks,
with only the first single-click being delivered to Gwenview.
By also zooming when double-clicking, for fast clicks each and every
click will result in an action, making zooming feel natural again.
Luckily there still won't be any duplicate clicks.