Propagate unused mouse wheel events in edit tools

Summary:
If an edit tool like `Crop` or `Red Eye Reduction` is active, zooming
and panning by mouse wheel does not work. This is caused by the
`QEvent::accepted` flag set to `true` by default and therefore not
propagating the event.
This patch clears the `accepted` flag for mouse wheel events until
a subclass of `AbstractRasterImageViewTool` will accept wheel events
by itself.

FIXED-IN: 18.08.0

Test Plan:
# Open Gwenview in View Mode
# Enter `Crop` or `Red Eye Reduction` tool
# Check if {key Ctrl wheel} zooming works
# Check if panning with {key wheel}/{key Alt wheel} works

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14286