window: Don't assume 640x480 max default size

window: Don't assume 640x480 max default size

Instead, use the monitor's work area.

This might have unforseen side effects that warrant a later revert, such
as:
- Apparently some WMs assume maximizing when a window is maximum screen
  size.
- WMs might not shrink the window by the decorations' size when it tries
  to be fullscreen.
- Applications might have buggy size request code that causes weirdly
  sized windows.