Avoid a indirect gtksourceviewmm include

Because it currently causes so many warnings about #warning (in the
gtksourceview headers) being a GNU-only extension.

glom/appwindow.cc

@@ -46,6 +46,7 @@
 #include <giomm/menu.h>
 
 #include <gtkmm/main.h>
+#include <gtkmm/textview.h>
 
 #include <giomm/file.h>
 #include <glibmm/spawn.h>

glom/mode_design/fields/box_db_table_definition.cc

@@ -19,6 +19,7 @@
  */
 
 #include "box_db_table_definition.h"
+#include <glom/mode_design/fields/dialog_fielddefinition.h>
 #include <glom/frame_glom.h>
 #include <glom/glade_utils.h>
 #include <glom/utils_ui.h> //For bold_message()).

glom/mode_design/fields/box_db_table_definition.h

@@ -22,12 +22,15 @@
 #define GLOM_MODE_DESIGN_BOX_DB_TABLE_DEFINITION_H
 
 #include <glom/box_db_table.h>
-#include <glom/mode_design/fields/dialog_fielddefinition.h>
 #include <glom/mode_design/fields/dialog_defaultformatting.h>
 
 namespace Glom
 {
 
+// Avoid including dialog_fielddefinition,
+// which includes some messy gtksourceview headers.
+class Dialog_FieldDefinition;
+
 class Box_DB_Table_Definition : public Box_DB_Table
 {
 public:

glom/mode_design/fields/dialog_fielddefinition.cc

@@ -23,6 +23,7 @@
 #include <glom/glade_utils.h>
 #include <glom/utils_ui.h>
 #include <glom/appwindow.h>
+#include <glom/mode_design/fields/dialog_fielddefinition.h>
 #include "../../box_db_table.h"
 #include <libglom/db_utils.h>
 //#include <libgnome/gnome-i18n.h>