app: add GimpToolWidget::hit() virtual function

... which takes the same arguments as GimpToolWidget::hover(), and
performs a hit-test, returning one of the following values:

  - GIMP_HIT_DIRECT:  The point corresponds to one of the widget's
    elements directly.

  - GIMP_HIT_INDIRECT:  The point does not correspond to one of the
    widget's elements directly, but the widget otherwise responds
    to press events at this point.

  - GIMP_HIT_NONE:  The widget does not respond to press events at
    this point.

Unlike hover(), hit() should not have any side effects.

(cherry picked from commit 385203f4d6)

app/display/display-enums.h

@@ -220,4 +220,17 @@ typedef enum
 } GimpZoomFocus;
 
 
+/*
+ * non-registered enums; register them if needed
+ */
+
+
+typedef enum  /*< pdb-skip, skip >*/
+{
+  GIMP_HIT_NONE,
+  GIMP_HIT_INDIRECT,
+  GIMP_HIT_DIRECT
+} GimpHit;
+
+
 #endif /* __DISPLAY_ENUMS_H__ */

app/display/gimptoolwidget.c

@@ -804,6 +804,22 @@ gimp_tool_widget_motion (GimpToolWidget   *widget,
                                                  coords, time, state);
 }
 
+GimpHit
+gimp_tool_widget_hit (GimpToolWidget   *widget,
+                      const GimpCoords *coords,
+                      GdkModifierType   state,
+                      gboolean          proximity)
+{
+  g_return_val_if_fail (GIMP_IS_TOOL_WIDGET (widget), GIMP_HIT_NONE);
+  g_return_val_if_fail (coords != NULL, GIMP_HIT_NONE);
+
+  if (GIMP_TOOL_WIDGET_GET_CLASS (widget)->hit)
+    return GIMP_TOOL_WIDGET_GET_CLASS (widget)->hit (widget,
+                                                     coords, state, proximity);
+
+  return GIMP_HIT_NONE;
+}
+
 void
 gimp_tool_widget_hover (GimpToolWidget   *widget,
                         const GimpCoords *coords,

app/display/gimptoolwidget.h

@@ -87,6 +87,10 @@ struct _GimpToolWidgetClass
                                 guint32                time,
                                 GdkModifierType        state);
 
+  GimpHit  (* hit)             (GimpToolWidget        *widget,
+                                const GimpCoords      *coords,
+                                GdkModifierType        state,
+                                gboolean               proximity);
   void     (* hover)           (GimpToolWidget        *widget,
                                 const GimpCoords      *coords,
                                 GdkModifierType        state,
@@ -252,6 +256,10 @@ void       gimp_tool_widget_motion          (GimpToolWidget        *widget,
                                              guint32                time,
                                              GdkModifierType        state);
 
+GimpHit    gimp_tool_widget_hit             (GimpToolWidget        *widget,
+                                             const GimpCoords      *coords,
+                                             GdkModifierType        state,
+                                             gboolean               proximity);
 void       gimp_tool_widget_hover           (GimpToolWidget        *widget,
                                              const GimpCoords      *coords,
                                              GdkModifierType        state,