canvas-container: remove duplicated code
canvas-container: remove commented out code
file-management-properties: remove unused function
file: remove unused code

We're repurposing the same function name for something different in the
next commit.
file: remove an incorrect comment
canvas-container: pre-compute selection

There's no need to always iterate over the list of icons in order to get
the selection - just keep a separate sorted list.
file: keep scaled thumbnail in cache

Or we'll constantly re-scale it every time a client asks for the icon.
file: add an utility to set the file directory

We're going to add some other code in here.
file: use a collation key for comparing directory names

Or we'll have to redo utf8 validation/normalization every time we add a
file into the directory, when we sort files that way.
canvas-container: don't iterate over all the files twice in a row

Merge the two loops in one.
canvas-container: cache icon positions for accessibles

Instead of calling g_list_index() every time, cache positions when
resorting.