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/vim.h b/src/vim.h
index c5e5751..d5518f8 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1816,6 +1816,7 @@
 #include "structs.h"	    /* file that defines many structures */
 
 #include "alloc.h"
+#include "beval.h"
 
 /* Values for "do_profiling". */
 #define PROF_NONE	0	/* profiling not started */