Alternate Mouse Capture mode [for Windowed]

[Tarvis]

UFO Extender has a mouse capture mode for windowed mode that releases the mouse if a menu or the geoscape is open. That way the only time mouse is captured is in the battlescape (with no menu open) where mouse scrolling is needed.

Currently OpenXcom just captures the mouse at all times, which makes it very hard to move the window around (must alt+tab out, then pray your mouse doesn't enter the actual game window or it will jump to the center)

Could be something like CaptureMouse=2 or so