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/gui.c b/src/gui.c
index 926750c..8118ca1 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -738,7 +738,7 @@
 	 * resized. */
 	win_new_shellsize();
 
-#ifdef FEAT_BEVAL
+#ifdef FEAT_BEVAL_GUI
 	/* Always create the Balloon Evaluation area, but disable it when
 	 * 'ballooneval' is off. */
 	if (balloonEval != NULL)