Fix thumbnail bar update after editing a viewed image

Summary:
After editing an image (e.g. rotate) a new thumbnail is created but
it is not repainted in the View Mode thumbnail bar.
The custom signals `documentChanged` and `busyStateChanged` provide
a `QModelIndex` for the modified document but this is based on the
source model and cannot be used in `ThumbnailBarView` where the list
view is based on a filtered proxy model and the index is wrong or
does not exist in some cases.
This patch uses the QUrl as argument for both signals and gets the
proper index for the current model in the receiver view.

FIXED-IN: 18.04.0

Test Plan:
* Open image in Gwenview View Mode
* Enable thumbnail bar ({key Ctrl B})
* Edit image (e.g. rotate with {key Ctrl R})
* Edit a second time (same or another image)
* Thumbnail should always correspond to edited image

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx, huoni

Differential Revision: https://phabricator.kde.org/D11714