resizeable frameless windows about nw

[capr]

X11 part blocked by #24

[capr]

update: WM_NCHITTEST ignores resize-related HT* constants on windows without WS_SIZEBOX and setting WS_SIZEBOX puts a border around the window until it is first resized, so that doesn't work either. looks like it's manual resizing for Windows too.

[capr]

Windows part done.

X11 part needs a way to query mouse state in screen coordinates regardless of where the window is. This is needed because of the async nature of X which screws everything up, making it impossible to drag a window by its top/left corner/margins if the mouse coordinates are given relative to the window.

[capr]

X11 part done.
OSX part done.

Now the whole thing happens in nw.lua (run nw_demo.lua to see).

TODO: apply constraints before moving/resizing otherwise we get strange behavior on all platforms (#64).