W32: correctly guess max window size from the size of the workarea of the screen...

W32: correctly guess max window size from the size of the workarea of the screen it's on

Get monitor on which the most of the window is located (nearest monitor if
window is not on screen), get its work area (area not occupied by taskbar or
any other bars) and use that for maxsize.

Previous default of 30000 meant that windows maximized onto full screen,
even covering the area where taskbar is.

https://bugzilla.gnome.org/show_bug.cgi?id=726592