Install icons as io.github.Hexchat

This matches our app-id as the desktop-file spec recommends.

This also fixes a bug where our notifications referred to this new
name already.

data/icons/meson.build

@@ -1,9 +1,11 @@
 icondir = join_paths(get_option('datadir'), 'icons/hicolor')
 install_data(
   'hexchat.png',
+  rename: 'io.github.Hexchat.png',
   install_dir: join_paths(icondir, '48x48/apps')
 )
 install_data(
   'hexchat.svg',
+  rename: 'io.github.Hexchat.svg',
   install_dir: join_paths(icondir, 'scalable/apps')
 )