Start implement druid layout component

druid/const.lua

@@ -40,6 +40,7 @@ M.ON_FOCUS_LOST = "on_focus_lost"
 M.ON_FOCUS_GAINED = "on_focus_gained"
 M.ON_LAYOUT_CHANGE = "on_layout_change"
 M.ON_MESSAGE_INPUT = "on_message_input"
+M.ON_WINDOW_RESIZED = "on_window_resized"
 M.ON_LANGUAGE_CHANGE = "on_language_change"
 
 
@@ -83,6 +84,14 @@ M.REVERSE_PIVOTS = {
 }
 
 
+M.LAYOUT_MODE = {
+	STRETCH_X = "stretch_x",
+    STRETCH_Y = "stretch_y",
+    FIT = gui.ADJUST_FIT,
+    STRETCH = gui.ADJUST_STRETCH,
+}
+
+
 M.VECTOR_ZERO = vmath.vector3(0)
 M.VECTOR_ONE = vmath.vector3(1)
 M.SYS_INFO = sys.get_sys_info()