[lazarus] mouse fixed

[Peter Vreman]

I've fixed the mouse which was not correctly read. Also the mousewheel is
supported. But that only works under linux, for win32 we should ask it to
the gimp-win32 list which i'm not subscribe to.

While implementing it, i came across the fact that we currently have other
Do...() functions with only the message, while delphi got them already
with normal parameters. I how far do we need to be delphi compatible in
such ways and the creating new private message functions which then call
the Do...() functions like delphi.

Peter