app: add GimpToolFocus tool widget

Add a new GimpToolFocus tool widget, which defines a focus region,
consisting of an inner limit, an outer limit, and a transition
midpoint between them.  The widget allows controlling the limits
and the midpoint, and moving, scaling, and rotating the region.

(cherry picked from commit 5e005a762c)

app/display/Makefile.am

@@ -182,6 +182,8 @@ libappdisplay_a_sources = \
 	gimptoolgui.h				\
 	gimptoolcompass.c			\
 	gimptoolcompass.h			\
+	gimptoolfocus.h				\
+	gimptoolfocus.c				\
 	gimptoolgyroscope.c			\
 	gimptoolgyroscope.h			\
 	gimptoolhandlegrid.c			\

app/display/gimptoolfocus.h

@@ -0,0 +1,58 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * gimptoolfocus.h
+ * Copyright (C) 2020 Ell
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_TOOL_FOCUS_H__
+#define __GIMP_TOOL_FOCUS_H__
+
+
+#include "gimptoolwidget.h"
+
+
+#define GIMP_TYPE_TOOL_FOCUS            (gimp_tool_focus_get_type ())
+#define GIMP_TOOL_FOCUS(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), GIMP_TYPE_TOOL_FOCUS, GimpToolFocus))
+#define GIMP_TOOL_FOCUS_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), GIMP_TYPE_TOOL_FOCUS, GimpToolFocusClass))
+#define GIMP_IS_TOOL_FOCUS(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GIMP_TYPE_TOOL_FOCUS))
+#define GIMP_IS_TOOL_FOCUS_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GIMP_TYPE_TOOL_FOCUS))
+#define GIMP_TOOL_FOCUS_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), GIMP_TYPE_TOOL_FOCUS, GimpToolFocusClass))
+
+
+typedef struct _GimpToolFocus        GimpToolFocus;
+typedef struct _GimpToolFocusPrivate GimpToolFocusPrivate;
+typedef struct _GimpToolFocusClass   GimpToolFocusClass;
+
+struct _GimpToolFocus
+{
+  GimpToolWidget        parent_instance;
+
+  GimpToolFocusPrivate *priv;
+};
+
+struct _GimpToolFocusClass
+{
+  GimpToolWidgetClass  parent_class;
+};
+
+
+GType            gimp_tool_focus_get_type (void) G_GNUC_CONST;
+
+GimpToolWidget * gimp_tool_focus_new      (GimpDisplayShell *shell);
+
+
+#endif /* __GIMP_TOOL_FOCUS_H__ */

po/POTFILES.in

@@ -262,6 +262,7 @@ app/display/gimpdisplayshell-utils.c
 app/display/gimpnavigationeditor.c
 app/display/gimpstatusbar.c
 app/display/gimptoolcompass.c
+app/display/gimptoolfocus.c
 app/display/gimptoolgyroscope.c
 app/display/gimptoolhandlegrid.c
 app/display/gimptoolline.c