tabline: Add %[] atom to the tabline, for random commands on click

Currently untested and undocumented.

src/nvim/screen.h

@@ -16,6 +16,29 @@
 #define NOT_VALID               40  /* buffer needs complete redraw */
 #define CLEAR                   50  /* screen messed up, clear it */
 
+/// Status line click definition
+typedef struct {
+  enum {
+    kStlClickDisabled = 0,   ///< Clicks to this area are ignored.
+    kStlClickTabSwitch,      ///< Switch to the given tab.
+    kStlClickTabClose,       ///< Close given tab.
+    kStlClickCmd,            ///< Run VimL command.
+  } type;     ///< Type of the click.
+  int tabnr;  ///< Tab page number.
+  char *cmd;  ///< Command to execute.
+} StlClickDefinition;
+
+/// Used for tabline clicks
+typedef struct {
+  StlClickDefinition def;  ///< Click definition.
+  const char *start;       ///< Location where region starts.
+} StlClickRecord;
+
+/// Array defining what should be done when tabline is clicked
+extern StlClickDefinition *tab_page_click_defs;
+
+/// Size of the tab_page_click_defs array
+extern long tab_page_click_defs_size;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "screen.h.generated.h"