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/misc1.c

@@ -3345,6 +3345,7 @@ is_mouse_key(int c)
 	|| c == K_LEFTDRAG
 	|| c == K_LEFTRELEASE
 	|| c == K_LEFTRELEASE_NM
+	|| c == K_MOUSEMOVE
 	|| c == K_MIDDLEMOUSE
 	|| c == K_MIDDLEDRAG
 	|| c == K_MIDDLERELEASE