floats: implement floating windows

Co-Author: Dongdong Zhou <dzhou121@gmail.com>

src/nvim/popupmnu.h

@@ -1,6 +1,8 @@
 #ifndef NVIM_POPUPMNU_H
 #define NVIM_POPUPMNU_H
 
+#include "nvim/macros.h"
+#include "nvim/grid_defs.h"
 #include "nvim/types.h"
 
 /// Used for popup menu items.
@@ -11,6 +13,7 @@ typedef struct {
   char_u *pum_info;        // extra info
 } pumitem_T;
 
+EXTERN ScreenGrid pum_grid INIT(= SCREEN_GRID_INIT);
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "popupmnu.h.generated.h"