api/ui: use ui options instead of one method per feature

Use new nvim_ui_ prefix to avoid breaking change.

src/nvim/ui.h

@@ -15,7 +15,7 @@ typedef struct {
 typedef struct ui_t UI;
 
 struct ui_t {
-  bool rgb;
+  bool rgb, pum_external;
   int width, height;
   void *data;
   void (*resize)(UI *ui, int rows, int columns);