[Lazarus] Working with threads and GUI

[Michael Schnell]

The LCL provides the function QueuAsyncCall() to have a thread execute a 
code snippet in the main thread and thus allow for GUI work.

Other than TThread.Synchronize, this does not stall the thread until the 
main thread activity is finished (which can take forever), but the 
procedure only gets queued, the thread just continues and the queued 
procedure gets executed when the main thread has time to run it.

-Michael