wxTimer::Start() only works from the main thread: mention it in the docs and add an assert in the code

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17475 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/timer.tex

@@ -20,6 +20,8 @@ In any case, you must start the timer with \helpref{Start}{wxtimerstart}
 after constructing it before it actually starts sending notifications. It can
 be stopped later with \helpref{Stop}{wxtimerstop}.
 
+{\bf NB:} note that timer can only be used from the main thread currently.
+
 \wxheading{Derived from}
 
 \helpref{wxObject}{wxobject}

src/common/timercmn.cpp

@@ -36,6 +36,7 @@
 #ifndef WX_PRECOMP
     #include "wx/intl.h"
     #include "wx/log.h"
+    #include "wx/thread.h"
 #endif
 
 #include "wx/timer.h"
@@ -131,6 +132,13 @@ void wxTimerBase::Notify()
 
 bool wxTimerBase::Start(int milliseconds, bool oneShot)
 {
+    // under MSW timers only work when they're started from the main thread so
+    // let the caller know about it
+#ifdef wxUSE_THREADS
+    wxASSERT_MSG( wxThread::IsMain(),
+                  _T("timer can only be started from the main thread") );
+#endif // wxUSE_THREADS
+
     if ( IsRunning() )
     {
         // not stopping the already running timer might work for some