MenuApplets: Use unveil()

The Clock and Audio applets really only need ("/res", "r") for LibGUI.

The CPUGraph applet also needs ("/proc/all", "r") for reading the CPU
usage data. Somewhat surprisingly, this also adds ("/etc/passwd", "r")
since CProcessStatisticsReader does username lookups.

MenuApplets/Clock/main.cpp

@@ -111,5 +111,12 @@ int main(int argc, char** argv)
     window->set_main_widget(widget);
     window->show();
 
+    if (unveil("/res", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    unveil(nullptr, nullptr);
+
     return app.exec();
 }