Convert the code for the multiwindow mode internal window manager to xcb

xcb conversion avoids xlib/xserver namespace collision and _XSERVER64 type
sizing issues

v2: Various fixes
v3: Don't include X11/extensions/windowswmstr.h, which uses the Display type
and thus depends on Xlib.h, just for _WINDOWSWM_NATIVE_HWND
v4: Fix indentation, add some error handling.
Fix a bug with ConfigureNotify handling
v5: Fix a bug which prevented WM_NORMAL_HINTS from being checked

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>