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/ProcessMemoryMapWidget.h

@@ -9,6 +9,8 @@
 
 #include <LibGUI/Widget.h>
 
+namespace SystemMonitor {
+
 class ProcessMemoryMapWidget final : public GUI::Widget {
     C_OBJECT(ProcessMemoryMapWidget);
 
@@ -25,3 +27,5 @@ private:
     pid_t m_pid { -1 };
     RefPtr<Core::Timer> m_timer;
 };
+
+}