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.cpp

@@ -11,6 +11,10 @@
 #include <LibGUI/SortingProxyModel.h>
 #include <LibGUI/TableView.h>
 
+REGISTER_WIDGET(SystemMonitor, ProcessFileDescriptorMapWidget)
+
+namespace SystemMonitor {
+
 ProcessFileDescriptorMapWidget::ProcessFileDescriptorMapWidget()
 {
     set_layout<GUI::VerticalBoxLayout>();
@@ -49,3 +53,5 @@ void ProcessFileDescriptorMapWidget::set_pid(pid_t pid)
     m_pid = pid;
     m_model->set_json_path(String::formatted("/proc/{}/fds", m_pid));
 }
+
+}