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/ProcessWindow.gml

@@ -0,0 +1,65 @@
+@GUI::Widget {
+    fill_with_background_color: true
+    layout: @GUI::VerticalBoxLayout {}
+
+    @GUI::Widget {
+        shrink_to_fit: true
+        layout: @GUI::HorizontalBoxLayout {
+            margins: [4]
+            spacing: 8
+        }
+
+        @GUI::Label {
+            name: "icon_label"
+            fixed_size: [32, 32]
+        }
+
+        @GUI::Label {
+            name: "process_name"
+            font_weight: "Bold"
+            text_alignment: "CenterLeft"
+            text: "This is the process name."
+        }
+    }
+
+    @GUI::HorizontalSeparator {
+        fixed_height: 2
+    }
+
+    @GUI::StackWidget {
+        name: "widget_stack"
+
+        @SystemMonitor::UnavailableProcessWidget {
+            name: "unavailable_process"
+        }
+
+        @GUI::TabWidget {
+            name: "available_process"
+
+            @SystemMonitor::ProcessStateWidget {
+                name: "process_state"
+                title: "State"
+            }
+
+            @SystemMonitor::ProcessMemoryMapWidget {
+                name: "memory_map"
+                title: "Memory map"
+            }
+
+            @SystemMonitor::ProcessFileDescriptorMapWidget {
+                name: "open_files"
+                title: "Open files"
+            }
+
+            @SystemMonitor::ProcessUnveiledPathsWidget {
+                name: "unveiled_paths"
+                title: "Unveiled paths"
+            }
+
+            @SystemMonitor::ThreadStackWidget {
+                name: "thread_stack"
+                title: "Stack"
+            }
+        }
+    }
+}