LibGUI: Move GUI::FocusPolicy to its own header & add explainer comment

Userland/Libraries/LibGUI/FocusPolicy.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/EnumBits.h>
+
+namespace GUI {
+
+// FocusPolicy determines how GUI widgets gain focus.
+//
+// - NoFocus:     The widget is not focusable.
+// - TabFocus:    The widget can gain focus by cycling through focusable widgets with the Tab key.
+// - ClickFocus:  The widget gains focus when clicked.
+// - StrongFocus: The widget can gain focus both via Tab, and by clicking on it.
+enum class FocusPolicy {
+    NoFocus = 0,
+    TabFocus = 0x1,
+    ClickFocus = 0x2,
+    StrongFocus = TabFocus | ClickFocus,
+};
+
+AK_ENUM_BITWISE_OPERATORS(FocusPolicy)
+
+}

Userland/Libraries/LibGUI/Widget.h

@@ -11,6 +11,7 @@
 #include <AK/String.h>
 #include <LibCore/Object.h>
 #include <LibGUI/Event.h>
+#include <LibGUI/FocusPolicy.h>
 #include <LibGUI/Forward.h>
 #include <LibGUI/Margins.h>
 #include <LibGfx/Color.h>
@@ -45,15 +46,6 @@ enum class VerticalDirection {
     Down
 };
 
-enum class FocusPolicy {
-    NoFocus = 0,
-    TabFocus = 0x1,
-    ClickFocus = 0x2,
-    StrongFocus = TabFocus | ClickFocus,
-};
-
-AK_ENUM_BITWISE_OPERATORS(FocusPolicy)
-
 enum class AllowCallback {
     No,
     Yes