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.cpp

@@ -14,6 +14,10 @@
 #include <LibGUI/TableView.h>
 #include <LibGfx/Palette.h>
 
+REGISTER_WIDGET(SystemMonitor, ProcessMemoryMapWidget)
+
+namespace SystemMonitor {
+
 class PagemapPaintingDelegate final : public GUI::TableCellPaintingDelegate {
 public:
     virtual ~PagemapPaintingDelegate() override = default;
@@ -121,3 +125,5 @@ void ProcessMemoryMapWidget::refresh()
     if (m_pid != -1)
         m_json_model->invalidate();
 }
+
+}