Always be explicit when emitting a signal by calling emit()

Mostly the code is explicit and calls the emit() method when emitting a
signal [1], like this:
    signal_name.emit();

However there are a few cases which use the function call operator on
the signal object [2], like this:
    signal_name();

The behaviour is identical [3] but it is preferred to be explicit that a
signal callback is being initiated, and it also makes them much easier
to search for too.

[1] List explicit emit() signal calls
        fgrep '.emit(' src/*.cc

[2] List function call operator emitted signals
        egrep "`sed -n '/sigc::signal/s/.*sigc::signal.*> *\([a-zA-Z_]*\).*/\1/p' include/*.h | tr '\n' '|' | sed 's/\(.*\).$/[^a-zA-Z_](\1)\\\(/'`" src/*.cc

[3] Quote from the libsigc++ Reference Manual, class sigc::signal
    https://developer.gnome.org/libsigc++/stable/classsigc_1_1signal7.html#ab37db0ecc788824d0baa3c301efc8dcd

        result_type sigc::signal<...>::operator()(...)

        Triggers the emission of the signal (see emit())

src/Dialog_Progress.cc

@@ -339,7 +339,7 @@ void Dialog_Progress::on_cancel()
 				sigc::mem_fun(*this, &Dialog_Progress::cancel_timeout), 1000 );
 		}
 		else cancelbutton->set_label( _("Force Cancel") );
-		operations[t]->operation_detail.signal_cancel( cancel );
+		operations[t]->operation_detail.signal_cancel.emit( cancel );
 		cancel = true;
 	}
 }

src/OperationDetail.cc

@@ -190,7 +190,7 @@ void OperationDetail::cancel( bool force )
 		cancelflag = 2;
 	else
 		cancelflag = 1;
-	signal_cancel(force);
+	signal_cancel.emit( force );
 }
 
 ProgressBar & OperationDetail::get_progressbar() const