Instead of calling out to a separate tools, I can use the `find` and
`install` programs to easily install the files to the right place.
I also take this opportunity to update the emmental.desktop file for v3,
including filling out as many audio-related mime types as I can find and
using the `desktop-file-install` command to not only install the file
but properly set the "Exec" and "Icon" fields based on the PREFIX=
passed to `make`
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>