Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/ui.h

@@ -1,29 +1,7 @@
 #ifndef NVIM_UI_H
 #define NVIM_UI_H
 /* ui.c */
-void ui_write(char_u *s, int len);
-int ui_inchar(char_u *buf, int maxlen, long wtime, int tb_change_cnt);
-int ui_char_avail(void);
-void ui_delay(long msec, int ignoreinput);
-void ui_suspend(void);
-int ui_get_shellsize(void);
-void ui_set_shellsize(int mustset);
-void ui_breakcheck(void);
-int vim_is_input_buf_full(void);
-int vim_is_input_buf_empty(void);
-char_u *get_input_buf(void);
-void set_input_buf(char_u *p);
-void add_to_input_buf(char_u *s, int len);
-void add_to_input_buf_csi(char_u *str, int len);
-void trash_input_buf(void);
-int read_from_input_buf(char_u *buf, long maxlen);
-void fill_input_buf(int exit_on_error);
-void read_error_exit(void);
-void ui_cursor_shape(void);
-int check_col(int col);
-int check_row(int row);
-int jump_to_mouse(int flags, int *inclusive, int which_button);
-int mouse_comp_pos(win_T *win, int *rowp, int *colp, linenr_T *lnump);
-win_T *mouse_find_win(int *rowp, int *colp);
-void im_save_status(long *psave);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "ui.h.generated.h"
+#endif
 #endif /* NVIM_UI_H */