add nova-plugin: in-process cxx-qt stats plugin replacing nova-stats subprocess

plugin/build.rs

@@ -0,0 +1,7 @@
+use cxx_qt_build::{CxxQtBuilder, PluginType, QmlModule};
+
+fn main() {
+    CxxQtBuilder::new_qml_module(QmlModule::new("NovaStats").plugin_type(PluginType::Dynamic))
+        .files(["src/system_stats.rs", "src/cpu_service.rs"])
+        .build();
+}