Fix BaseUI x-thread signal vs thread-start race

Control surfaces c'tor usually subscribe to signals e.g.
PortConnectedOrDisconnected. This happens after the parent BaseUI
is created, but before set_active() -> BaseUI::run() is called.

At this point in time there is no run_loop thread.

There are two options to handle AbstractUI::call_slot():

 A. Queue the event in the event-loop, using the thread-local
    request buffer of the caller. Then hope the BaseUI
    thread is started, and calls ::handle_ui_requests() before
    the memory pool runs out of space.

 B. Handle the event in the calling thread. -- This may not be
    rt-safe and may call functions with locks held by the caller.
    It will however not accumulate events.

This takes approach (B). If _run_loop_thread is NULL, directly
handle the signal.

In the past, prior to 50abcc74b5, approach (A) was taken.
NULL never matched Glib::Threads::Thread::self().

This also reverts a prior attempt (e417495505) to address this issue.

libs/pbd/base_ui.cc

@@ -119,12 +119,11 @@ BaseUI::run ()
 
 	m_context = MainContext::create();
 	_main_loop = MainLoop::create (m_context);
+	attach_request_source ();
 
 	Glib::Threads::Mutex::Lock lm (_run_lock);
 	_run_loop_thread = PBD::Thread::create (boost::bind (&BaseUI::main_thread, this));
 	_running.wait (_run_lock);
-
-	attach_request_source ();
 }
 
 void