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/getchar.c b/src/getchar.c
index 2e91c24..aa65a8c 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -1792,7 +1792,7 @@
      */
     may_garbage_collect = FALSE;
 #endif
-#ifdef FEAT_BEVALTERM
+#ifdef FEAT_BEVAL_TERM
     if (c != K_MOUSEMOVE && c != K_IGNORE)
     {
 	/* Don't trigger 'balloonexpr' unless only the mouse was moved. */