Added:
- AMScanThumbnailView::setSelectionBetween() ~ Sets all the items
between the two passed model indices as selected using the passed
selection command

Altered:
- AMScanThumbnailView::mouseReleaseEvent() ~ Now, if the shift button is
held down but the user is not dragging a box for the selection, the view
iterates backwards to the previously selected item, and then selects all
model indices between the two.

- AMScanThumbnailView::mousePressEvent() ~ No longer calls base version.
QAbstractItemView was doing too much logic on its own back, which was
interfering with the required logic that we need from the thumbnail
view. This means that:

Still ToDo:
- Need to rewrite double-click logic, as this was one of the things that
the QAbstractItemView was doing for us (amongst other more annoying
things).
Added:
- AMScanThumbnailView::doubleClickDelay_ ~ How close together (ms) clicks have to be to consider them double-clicks
- AMScanThumbnailView::doubleClickTimer_ ~ QTimer used to measure how close clicks are together

Altered:
- AMScanThumbnailView::mouseReleaseEvent() ~ Now checks if this click was the second of a double-click, emits doubleClicked if was
- AMScanThumbnailView & AMScanDataView ~ Added a bunch of debugs, will be removed when Issue complete
Removed:
- QDebugs that I'd used for testing in AMScanDataView and
AMScanThumbnailView

Altered:
- Click delay for double-click now set at 500ms (Windows standard)
Merge pull request #940 from acquaman/Issue879

Pull request for Issue879: Shift click selection not working on AMScanThumbnailView