Allow dragging ingameWindows with middle mouse button

[Farmer-Markus]

Just a small idea to let users drag ingame windows with the middle mouse button.
Very useful when playing on a laptop

[Flow86]

Useful extension, especially since you do not need to find the (relatively) small title bar to move the window around.

Approved

[Flamefire]

Thanks, looks useful indeed.
Just 2 minor comments to help maintainability. Not fully sure about the return value (3rd comment) but looks odd to unconditionally return false, even though I'm aware we aren't consistent with those.

[Spikeone]

I still think this should be an option you can disable - at least that way I hope that people who, for whatever reason don't want this, aren't bothered and maybe it's a bit more prominent to find and actually know that it is possible at all.

[Flamefire]

IMO it is so unintrusive we don't need an option: Zoom is the only other use for the mousewheel we have so I doubt people will unintentionally use this
Not sure if we have documentation for the zoom function, but if we could add this there. If not we should likely add both

[Flamefire]

Small bug in WinAPI and small suggestions. Would also merge without those though.