ui: use line-based rather than char-based updates in screen.c

Add ext_newgrid and ext_hlstate extensions. These use predefined
highlights and line-segment based updates, for efficiency and
simplicity.. The ext_hlstate extension in addition allows semantic
identification of builtin and syntax highlights.

Reimplement the old char-based updates in the remote UI layer, for
compatibility. For the moment, this is still the default. The bulitin
TUI uses the new line-based protocol.

cmdline uses curwin cursor position when ext_cmdline is active.

src/nvim/ugrid.h

@@ -7,7 +7,7 @@
 typedef struct ucell UCell;
 typedef struct ugrid UGrid;
 
-#define CELLBYTES (4 * (MAX_MCO+1))
+#define CELLBYTES (sizeof(schar_T))
 
 struct ucell {
   char data[CELLBYTES + 1];
@@ -17,7 +17,6 @@ struct ucell {
 struct ugrid {
   int top, bot, left, right;
   int row, col;
-  HlAttrs clear_attrs;
   int width, height;
   HlAttrs attrs;
   UCell **cells;