Sort images by filename in active-imageviewer. It's static sorting, that
is, newer images will be appended to grid view end. Reopen imageviewer
to sort them again.
I think we can implement dynamic sorting using QSortFilterProxyModel as
the base class of MetadataModel instead of AbstractMetadataModel.
The sorting is case sensitive too. I have not figure out how to solve
this yet.


(cherry picked from commit 3531ed4599c2fb9e2ff3440f078e09dc5f0c132d)