improve GUI behavior of the tempo dialog by preselecting the start bpm text field

most common user action will be: show dialog, type keys, press return. This works now.

gtk2_ardour/tempo_dialog.cc

@@ -276,6 +276,9 @@ TempoDialog::init (const Timecode::BBT_Time& when, double bpm, double end_bpm, d
 
 	tempo_type_change();
 
+	bpm_spinner.select_region (0, -1);
+	bpm_spinner.grab_focus ();
+
 	tapped = false;
 }