Fixed grid.cr compilation errors

Align_x and align_y were being interpreted as symbols. They're now specified as Aligns.
@this was returning an error because it's supposed to be a Control*, not a Grid*. It's now casted using ui_control().
This patch compiles the grid_gallery.cr successfully on Linux.

src/hedron/ui/grid.cr

@@ -10,12 +10,12 @@ module Hedron
     property align_x : Align
     property align_y : Align
 
-    def initialize(@size, @expand, @align_x, @align_y); end
+    def initialize(@size, @expand, @align_x : Align, @align_y : Align); end
   end
 
   class Grid < Control
     def initialize
-      @this = UI.new_grid
+      @this = ui_control(UI.new_grid)
     end
 
     def initialize(@this); end