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/ProcessUnveiledPathsWidget.cpp

@@ -10,6 +10,11 @@
 #include <LibGUI/JsonArrayModel.h>
 #include <LibGUI/SortingProxyModel.h>
 #include <LibGUI/TableView.h>
+#include <LibGUI/Widget.h>
+
+REGISTER_WIDGET(SystemMonitor, ProcessUnveiledPathsWidget)
+
+namespace SystemMonitor {
 
 ProcessUnveiledPathsWidget::ProcessUnveiledPathsWidget()
 {
@@ -32,3 +37,5 @@ void ProcessUnveiledPathsWidget::set_pid(pid_t pid)
     m_pid = pid;
     m_model->set_json_path(String::formatted("/proc/{}/unveil", m_pid));
 }
+
+}