Improve experience when using mouse clicks to zoom

Summary:
Gwenview allows to {key Ctrl}-click to zoom in, and
{key 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.

Test Plan:
Hold {key Ctrl}, repeatedly press left and right mouse buttons (start
slowly and then increase clicking rate). Zooming should work reliably
now.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D14104