Currently, we just skipped the notification comparison, if the
notification had a raw icon attached. This is a bit counterintuitive.
Calculating a checksum of the raw icon's data is the solution.
For that we cache the pixel buffer and introduce a field, which saves
the current icon's id. The icon_id may be a path or a hash.
So you can compare two notifications by their icon_id field regardless
of their icon type by their icon_id field.
Cleans up the clutter in draw and since only a single icon function is
called externally from icon.c it'll make things easier if we ever decide
to switch icon libraries.
Rename both functions to make clear, what the difference between the
previous functions _from_file and _from_path is. Also remove the
superfluous `== NULL` checks as these don't match dunst's current style.
