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/keymap.h b/src/keymap.h
index 7cb5c69..8a34f0e 100644
--- a/src/keymap.h
+++ b/src/keymap.h
@@ -269,6 +269,7 @@
     , KE_NOP = 97		/* doesn't do something */
     , KE_FOCUSGAINED = 98	/* focus gained */
     , KE_FOCUSLOST = 99		/* focus lost */
+    , KE_MOUSEMOVE = 100	/* mouse moved with no button down */
 };
 
 /*
@@ -437,6 +438,7 @@
 #define K_LEFTDRAG	TERMCAP2KEY(KS_EXTRA, KE_LEFTDRAG)
 #define K_LEFTRELEASE	TERMCAP2KEY(KS_EXTRA, KE_LEFTRELEASE)
 #define K_LEFTRELEASE_NM TERMCAP2KEY(KS_EXTRA, KE_LEFTRELEASE_NM)
+#define K_MOUSEMOVE	TERMCAP2KEY(KS_EXTRA, KE_MOUSEMOVE)
 #define K_MIDDLEMOUSE	TERMCAP2KEY(KS_EXTRA, KE_MIDDLEMOUSE)
 #define K_MIDDLEDRAG	TERMCAP2KEY(KS_EXTRA, KE_MIDDLEDRAG)
 #define K_MIDDLERELEASE	TERMCAP2KEY(KS_EXTRA, KE_MIDDLERELEASE)