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/gui.c b/src/gui.c
index 7ef1c95..926750c 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -740,7 +740,10 @@
 
 #ifdef FEAT_BEVAL
 	/* Always create the Balloon Evaluation area, but disable it when
-	 * 'ballooneval' is off */
+	 * 'ballooneval' is off. */
+	if (balloonEval != NULL)
+	    vim_free(balloonEval);
+	balloonEvalForTerm = FALSE;
 # ifdef FEAT_GUI_GTK
 	balloonEval = gui_mch_create_beval_area(gui.drawarea, NULL,
 						     &general_beval_cb, NULL);