Add and install icons.

This adds one new app icon and one new symbolic icon, and makes meson install these to the appropriate
directories, and adds an appropriate entry in the .desktop file.

meson.build

@@ -25,6 +25,7 @@ cargo_script = find_program('build-aux/cargo.sh')
 prefix = get_option('prefix')
 bindir = prefix / get_option('bindir')
 datadir = prefix / get_option('datadir')
+iconsdir = datadir / 'icons'
 
 cargo_sources = files(
   'Cargo.toml',