api/ui: externalize tabline

- Work with a bool[] array parallel to the UIWidget enum.
- Rename some functions.
- Documentation.

src/nvim/ui.h

@@ -8,12 +8,13 @@
 #include "api/private/defs.h"
 #include "nvim/buffer_defs.h"
 
-// values for externalized widgets
 typedef enum {
+  kUICmdline = 0,
+  kUIPopupmenu,
   kUITabline,
-  kUICmdline,
-  kUIWildmenu
+  kUIWildmenu,
 } UIWidget;
+#define UI_WIDGETS (kUIWildmenu + 1)
 
 typedef struct {
   bool bold, underline, undercurl, italic, reverse;
@@ -23,7 +24,8 @@ typedef struct {
 typedef struct ui_t UI;
 
 struct ui_t {
-  bool rgb, pum_external, tabline_external;
+  bool rgb;
+  bool ui_ext[UI_WIDGETS];  ///< Externalized widgets
   int width, height;
   void *data;
   void (*resize)(UI *ui, int rows, int columns);