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.

src/message.c

@@ -1179,6 +1179,7 @@ wait_return(int redraw)
 				|| c == K_RIGHTDRAG  || c == K_RIGHTRELEASE
 				|| c == K_MOUSELEFT  || c == K_MOUSERIGHT
 				|| c == K_MOUSEDOWN  || c == K_MOUSEUP
+				|| c == K_MOUSEMOVE
 				|| (!mouse_has(MOUSE_RETURN)
 				    && mouse_row < msg_row
 				    && (c == K_LEFTMOUSE