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/evalfunc.c b/src/evalfunc.c
index bd5cf8d..c03e214 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5583,13 +5583,13 @@
 #ifdef FEAT_AUTOSERVERNAME
 	"autoservername",
 #endif
-#ifdef FEAT_BEVAL
+#ifdef FEAT_BEVAL_GUI
 	"balloon_eval",
 # ifndef FEAT_GUI_W32 /* other GUIs always have multiline balloons */
 	"balloon_multiline",
 # endif
 #endif
-#ifdef FEAT_BEVALTERM
+#ifdef FEAT_BEVAL_TERM
 	"balloon_eval_term",
 #endif
 #if defined(SOME_BUILTIN_TCAPS) || defined(ALL_BUILTIN_TCAPS)