LibGUI: Add forwarding header

This patch adds <LibGUI/Forward.h> and uses it a bunch.
It also dragged various header dependency reduction changes into it.

DevTools/HackStudio/CursorTool.h

@@ -28,12 +28,9 @@
 
 #include "Tool.h"
 #include <AK/HashMap.h>
+#include <LibGUI/Forward.h>
 #include <LibGfx/Point.h>
 
-namespace GUI {
-class Widget;
-}
-
 class CursorTool final : public Tool {
 public:
     explicit CursorTool(FormEditorWidget& editor)

DevTools/HackStudio/EditorWrapper.h

@@ -28,10 +28,6 @@
 
 #include <LibGUI/Widget.h>
 
-namespace GUI {
-class Label;
-}
-
 class Editor;
 
 class EditorWrapper : public GUI::Widget {

DevTools/HackStudio/FindInFilesWidget.h

@@ -28,12 +28,6 @@
 
 #include <LibGUI/Widget.h>
 
-namespace GUI {
-class Button;
-class TableView;
-class TextBox;
-}
-
 class FindInFilesWidget final : public GUI::Widget {
     C_OBJECT(FindInFilesWidget)
 public:

DevTools/HackStudio/FormEditorWidget.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/HashTable.h>
 #include <LibGUI/ScrollableWidget.h>
 
 class FormWidget;

DevTools/HackStudio/Locator.h

@@ -28,11 +28,6 @@
 
 #include <LibGUI/Widget.h>
 
-namespace GUI {
-class ModelIndex;
-class TableView;
-}
-
 class LocatorTextBox;
 
 class Locator final : public GUI::Widget {

DevTools/HackStudio/ProcessStateWidget.h

@@ -28,10 +28,6 @@
 
 #include <LibGUI/Widget.h>
 
-namespace GUI {
-class Label;
-}
-
 class ProcessStateWidget final : public GUI::Widget {
     C_OBJECT(ProcessStateWidget)
 public:

DevTools/HackStudio/ProjectFile.cpp

@@ -28,6 +28,11 @@
 #include <LibCore/File.h>
 #include <string.h>
 
+ProjectFile::ProjectFile(const String& name)
+    : m_name(name)
+{
+}
+
 const GUI::TextDocument& ProjectFile::document() const
 {
     if (!m_document) {

DevTools/HackStudio/ProjectFile.h

@@ -44,10 +44,7 @@ public:
     const GUI::TextDocument& document() const;
 
 private:
-    explicit ProjectFile(const String& name)
-        : m_name(name)
-    {
-    }
+    explicit ProjectFile(const String& name);
 
     String m_name;
     mutable RefPtr<GUI::TextDocument> m_document;

DevTools/HackStudio/Tool.h

@@ -27,16 +27,10 @@
 #pragma once
 
 #include <AK/Noncopyable.h>
+#include <LibGUI/Forward.h>
 
 class FormEditorWidget;
 
-namespace GUI {
-class KeyEvent;
-class MouseEvent;
-class PaintEvent;
-class Painter;
-}
-
 class Tool {
     AK_MAKE_NONCOPYABLE(Tool)
     AK_MAKE_NONMOVABLE(Tool)