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/ThreadStackWidget.cpp

@@ -9,9 +9,14 @@
 #include <LibCore/Timer.h>
 #include <LibGUI/BoxLayout.h>
 #include <LibGUI/Model.h>
+#include <LibGUI/Widget.h>
 #include <LibSymbolication/Symbolication.h>
 #include <LibThreading/BackgroundAction.h>
 
+REGISTER_WIDGET(SystemMonitor, ThreadStackWidget)
+
+namespace SystemMonitor {
+
 class ThreadStackModel final : public GUI::Model {
 
     enum Column {
@@ -127,3 +132,5 @@ void ThreadStackWidget::custom_event(Core::CustomEvent& event)
     auto& completion_event = verify_cast<CompletionEvent>(event);
     verify_cast<ThreadStackModel>(m_stack_table->model())->set_symbols(completion_event.symbols());
 }
+
+}