meson: Add appdata, desktop, and icons + install

This should close #3.

Once Calls is further in development adding proper release info and
screenshots would also be good.

data/icons/meson.build

+icondir = join_paths(get_option('datadir'), 'icons/hicolor')
+
+sizes = [ '16', '24', '32', '48', '64', '128', '256' ]
+
+foreach size : sizes
+    install_subdir(join_paths('hicolor', '@0@x@0@'.format(size)),
+        install_dir: icondir,
+    )
+endforeach

data/meson.build

@@ -24,5 +24,4 @@ if appstream_util.found()
 endif
 
 # Icons
-install_data('sm.puri.Calls.png',
-             install_dir : join_paths(datadir, 'icons', 'hicolor', '256x256', 'apps'))
+subdir('icons')
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