refactor: move tabline code to statusline.c (#21008)

* refactor: move tabline code to statusline.c

Problem:	Tabline code is closely related to statusline, but still left over in drawscreen.c and screen.c.
Solution:	Move it to statusline.c.

* refactor: add statusline_defs.h

src/nvim/grid_defs.h

@@ -110,24 +110,6 @@ struct ScreenGrid {
                            false, 0, 0, NULL, false, true, 0, \
                            0, 0, 0, 0, 0,  false }
 
-/// 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.
-    kStlClickFuncRun,       ///< Run user function.
-  } type;      ///< Type of the click.
-  int tabnr;   ///< Tab page number.
-  char *func;  ///< Function to run.
-} StlClickDefinition;
-
-/// Used for tabline clicks
-typedef struct {
-  StlClickDefinition def;  ///< Click definition.
-  const char *start;       ///< Location where region starts.
-} StlClickRecord;
-
 typedef struct {
   int args[3];
   int icell;