Reduce includes required for Selection

Previously a triggerbox_ui.h change triggered a complete UI
rebuild. This follows the concept of other editor_selection
lists which use forward declarations.

gtk2_ardour/audio_trigger_properties_box.h

@@ -26,6 +26,7 @@
 
 #include "ardour/ardour.h"
 #include "ardour/session_handle.h"
+#include "ardour/triggerbox.h"
 
 #include "widgets/ardour_button.h"
 

gtk2_ardour/editor_audio_import.cc

@@ -50,6 +50,7 @@
 #include "ardour/region_factory.h"
 #include "ardour/smf_source.h"
 #include "ardour/source_factory.h"
+#include "ardour/triggerbox.h"
 #include "ardour/utils.h"
 #include "pbd/memento_command.h"
 

gtk2_ardour/editor_ops.cc

@@ -127,6 +127,7 @@
 #include "timers.h"
 #include "transpose_dialog.h"
 #include "transform_dialog.h"
+#include "triggerbox_ui.h"
 #include "ui_config.h"
 #include "utils.h"
 #include "vca_time_axis.h"

gtk2_ardour/editor_rulers.cc

@@ -44,6 +44,7 @@
 #include "ardour/session.h"
 #include "ardour/tempo.h"
 #include "ardour/profile.h"
+#include "ardour/triggerbox.h"
 
 #include "temporal/tempo.h"
 

gtk2_ardour/midi_trigger_properties_box.h

@@ -23,6 +23,7 @@
 #include <gtkmm/label.h>
 
 #include "ardour/ardour.h"
+#include "ardour/triggerbox.h"
 
 #include "audio_trigger_properties_box.h"
 

gtk2_ardour/selection.cc

@@ -48,6 +48,7 @@
 #include "automation_time_axis.h"
 #include "public_editor.h"
 #include "control_point.h"
+#include "triggerbox_ui.h"
 #include "vca_time_axis.h"
 
 #include "pbd/i18n.h"

gtk2_ardour/selection_properties_box.cc

@@ -34,6 +34,7 @@
 #include "editor.h"
 #include "region_view.h"
 #include "time_info_box.h"
+#include "triggerbox_ui.h"
 
 #include "multi_region_properties_box.h"
 

gtk2_ardour/trigger_page.cc

@@ -49,6 +49,7 @@
 
 #include "trigger_page.h"
 #include "trigger_strip.h"
+#include "triggerbox_ui.h"
 #include "ui_config.h"
 #include "utils.h"
 

gtk2_ardour/trigger_selection.h

@@ -22,7 +22,7 @@
 
 #include <list>
 
-#include "triggerbox_ui.h"
+class TriggerEntry;
 
 class TriggerSelection : public std::list<TriggerEntry*>
 {