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/ex_cmds2.c b/src/ex_cmds2.c
index c6a6dbe..e2416057 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -1291,7 +1291,7 @@
     if (did_one)
 	redraw_after_callback(need_update_screen);
 
-#ifdef FEAT_BEVALTERM
+#ifdef FEAT_BEVAL_TERM
     if (bevalexpr_due_set)
     {
 	this_due = proftime_time_left(&bevalexpr_due, &now);