[Lazarus] LCL messaging system SendMessage and Multi-Threads

[Michael Schnell]

When I said that the Windows documentation of PostMessage can be used, I
did not think of the description of the messages to be sent by
PostMessage. Same might be Windows-specific and in the LCL not ported
(or portable) to Linux Widget Types like GTK. Here I suppose a
documentation is lacking and thus only "user message" can be used, that
you receive in your own Pascal code (e.g. by procedure ... message).

-Michael