patch 8.0.1309: cannot use 'balloonexpr' in a terminal

Problem:    Cannot use 'balloonexpr' in a terminal.
Solution:   Add 'balloonevalterm' and add code to handle mouse movements in a
            terminal. Initial implementation for Unix with GUI.
diff --git a/src/option.h b/src/option.h
index f9972f2..2084517 100644
--- a/src/option.h
+++ b/src/option.h
@@ -382,6 +382,9 @@
 EXTERN char_u	*p_bexpr;
 # endif
 #endif
+# ifdef FEAT_BEVALTERM
+EXTERN int	p_bevalterm;	/* 'balloonevalterm' */
+# endif
 #ifdef FEAT_BROWSE
 EXTERN char_u	*p_bsdir;	/* 'browsedir' */
 #endif