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/os_win32.c b/src/os_win32.c
index 7654c31..3a2b356 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -1469,7 +1469,7 @@
 		    dwWaitTime = 10;
 	    }
 #endif
-#ifdef FEAT_BEVAL
+#ifdef FEAT_BEVAL_GUI
 	    if (p_beval && dwWaitTime > 100)
 		/* The 'balloonexpr' may indirectly invoke a callback while
 		 * waiting for a character, need to check often. */