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.

CCBUG: 284672