Fix Next/Previous custom shortcuts behavior when using CTRL key
The selection mode for the ThumbnailView and ThumbnailBarView is set to QAbstractItemView::ExtendedSelection. When using shortcuts like Previous/Next/First/Last that involve the CTRL key, the appropriate index will be selected without clearing the current selection. Therefore, multiple elements will be selected and this leads to the undesired split view when requesting the new image.
With the changes we clear the current selection when the actions Previous/Next/First/Last are executed.