patch 8.0.1312: balloon_show() only works in terminal when compiled with GUI

Problem:    balloon_show() only works in terminal when compiled with the GUI.
Solution:   Add FEAT_BEVAL_GUI and refactor to move common code out of the GUI
            specific file.
diff --git a/src/globals.h b/src/globals.h
index c198664..b479579 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1649,7 +1649,7 @@
 EXTERN int  timer_busy INIT(= 0);   /* when timer is inside vgetc() then > 0 */
 #endif
 
-#ifdef FEAT_BEVALTERM
+#ifdef FEAT_BEVAL_TERM
 EXTERN int  bevalexpr_due_set INIT(= FALSE);
 EXTERN proftime_T bevalexpr_due;
 #endif