Add CallWindowProc to User32

[heldplayer]

Method documentation: https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-callwindowproca

First time contributing here, not sure what a test for this method would look like so any pointers would be appreciated ^^

[dbwiddis]

Thanks for your contribution, @heldplayer. A few requests:

• Please add a test case to exercise this function.
• Please use your full name in your commit message, rather than your github alias. You should be able to configure git or your IDE to do so using git config --global user.name "FIRST_NAME LAST_NAME".
• Please squash your commits once you've set the name info.
• I think the javadoc would read better without including the type info ([in] Type; HWND, etc.), as most users have the Windows API documentation available to them. Use <p> for paragraph breaks when needed.

[heldplayer]

I'm hoping the test location is adequate in User32WindowMessagesTest, as it already had pretty much everything set up to test the function. Otherwise I would have had to create a test case that would have copied a big part of it.

If you'd prefer me to go down that route instead, or have a better idea how I should implement this test, feel free to let me know :)

[dbwiddis]

Thanks, looks good to me. I'll wait a day in case anyone else has comments and then merge.