Blocking every window message blocks intentional user actions
like minimizing, closing(Alt-F4 doesn't work either), resizing etc.

This commit fixes that kind of problem by providing bitflag interface
to block certain predefined range of window messages.

To block every messages like before,
you can return `MessageFilter::all()` on `set_message_filter()`.

There was some perspective mistake during naming the method.
It's getter for pipeline, not setter for user code.

The method would(and maybe should) not be used in user code,
so make it protected seems good idea.

- Explanation about using MessageFilter is added.
- Wrap consts like `WM_USER` with backticks(`)

Harness doesn't seem to handle any input events,
so only thing we can simply test is `MessageFilter::WindowControl`