Uninstall the icons with "make uninstall".

Just in case not all shells expand {list}, create the apps/icons and
apps/mimetypes directories separately; that makes it a bit clearer in
any case.

Change-Id: I25b3426a17a2ca7e1ce3ce1f7f9e1c650db511a7
Reviewed-on: https://code.wireshark.org/review/5578
Reviewed-by: Guy Harris <guy@alum.mit.edu>