Implement job control

- Add a job control module for spawning and controlling co-processes
- Add three vimscript functions for interfacing with the module
- Use dedicated header files for typedefs/structs in event/job modules

src/vim.h

@@ -792,6 +792,7 @@ enum auto_event {
   EVENT_INSERTCHANGE,           /* when changing Insert/Replace mode */
   EVENT_INSERTENTER,            /* when entering Insert mode */
   EVENT_INSERTLEAVE,            /* when leaving Insert mode */
+  EVENT_JOBACTIVITY,            /* when job sent some data */
   EVENT_MENUPOPUP,              /* just before popup menu is displayed */
   EVENT_QUICKFIXCMDPOST,        /* after :make, :grep etc. */
   EVENT_QUICKFIXCMDPRE,         /* before :make, :grep etc. */
@@ -1304,6 +1305,7 @@ enum {
     VV_HLSEARCH,
     VV_OLDFILES,
     VV_WINDOWID,
+    VV_JOB_DATA,
     VV_LEN, /* number of v: vars */
 };