LibGUI+WindowServer: Introduce WindowModes

Previously, Windows only understood blocking modality: Windows were
either modal, i.e., in a blocking state, or not. Windows could also
be set as Accessories or ToolWindows, attributes which technically
applied modes to their parents but were implemented ad hoc. This patch
redefines these modal effects as WindowModes and sets up some helpers.
This will let us simplify a lot of modal logic in the upcoming patches
and make it easier to build new modal effects in the future.

Windows can now set 1 of 5 modes before reification:
-Modeless:	No modal effect; begins a new modal chain
-Passive:	Window joins its modal chain but has no effect
-RenderAbove:	Window renders above its parent
-CaptureInput:	Window captures the active input role from its parent
-Blocking:	Window blocks all interaction with its modal chain

States like fullscreen and tiling are dynamic and don't alter behavior
in modal chains, so they aren't included.

Userland/Libraries/LibGUI/Window.h

@@ -14,6 +14,7 @@
 #include <LibCore/Object.h>
 #include <LibGUI/FocusSource.h>
 #include <LibGUI/Forward.h>
+#include <LibGUI/WindowMode.h>
 #include <LibGUI/WindowType.h>
 #include <LibGfx/Forward.h>
 #include <LibGfx/Rect.h>
@@ -33,8 +34,8 @@ public:
     bool is_modified() const;
     void set_modified(bool);
 
-    bool is_modal() const { return m_modal; }
-    void set_modal(bool);
+    bool is_modal() const { return m_window_mode != WindowMode::Modeless; }
+    bool is_blocking() const { return m_window_mode == WindowMode::Blocking; }
 
     bool is_fullscreen() const { return m_fullscreen; }
     void set_fullscreen(bool);
@@ -71,6 +72,9 @@ public:
     WindowType window_type() const { return m_window_type; }
     void set_window_type(WindowType);
 
+    WindowMode window_mode() const { return m_window_mode; }
+    void set_window_mode(WindowMode);
+
     int window_id() const { return m_window_id; }
 
     void make_window_manager(unsigned event_mask);
@@ -284,12 +288,12 @@ private:
     Gfx::IntSize m_size_increment;
     Gfx::IntSize m_base_size;
     WindowType m_window_type { WindowType::Normal };
+    WindowMode m_window_mode { WindowMode::Modeless };
     AK::Variant<Gfx::StandardCursor, NonnullRefPtr<Gfx::Bitmap>> m_cursor { Gfx::StandardCursor::None };
     AK::Variant<Gfx::StandardCursor, NonnullRefPtr<Gfx::Bitmap>> m_effective_cursor { Gfx::StandardCursor::None };
     bool m_is_active_input { false };
     bool m_has_alpha_channel { false };
     bool m_double_buffering_enabled { true };
-    bool m_modal { false };
     bool m_resizable { true };
     bool m_obey_widget_min_size { true };
     Optional<Gfx::IntSize> m_resize_aspect_ratio {};