SystemMonitor: Move process window to GML

Extra stuff done in this commit to facilitate the above (if you want to
really push my commit count, ask for more atomicisation):
- Register a bunch of widgets that are used in the process window.
- Allow setting the pid after the fact for the process state widget.

Userland/Applications/SystemMonitor/ProcessFileDescriptorMapWidget.h

@@ -9,6 +9,8 @@
 
 #include <LibGUI/Widget.h>
 
+namespace SystemMonitor {
+
 class ProcessFileDescriptorMapWidget final : public GUI::Widget {
     C_OBJECT(ProcessFileDescriptorMapWidget);
 
@@ -24,3 +26,5 @@ private:
     RefPtr<GUI::JsonArrayModel> m_model;
     pid_t m_pid { -1 };
 };
+
+}