Added window_set_topmost, published code

This function does what you would get when using “Let the game window always stay on top” Global Game Settings option or window_set_stayontop function in older versions of GameMaker. In GameMaker: Studio it had been deprecated and there were seemingly no extensions to do this? Strange for something that is one line of WINAPI code,

SetWindowPos(hwnd, stayontop ? HWND_TOPMOST : HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);

Also published the extension's source code: https://bitbucket.org/yal_cc/window_commands