[Customdrawn] CustomDtawnPlatform

[Michael Schnell]

When setting the Lazarus build mode variable "Widget Type" = "fpGUI", 
same uses the variable "fpGUIPlatform" to define the backend to be used.

So I think the Widget Type "CustomDrawn" could use a variable 
"CustomDrawnPlatform" for this purpose.

As I can see a Form drawn by CustomDrawn, I suppose, here, somehow 
automatically a backend is used that successfully hooks to my 
installation. But I might want to do some cross development (e.g. for 
Android). So I) think such a variable does make sense.

-Michael