Handle clicks on grid edges normally when not using drag-resizing

Clicking on (or near) the grid column or row edges was handled specially
to allow dragging them in order to resize the column or row, but this
doesn't need to be done if drag-resizing the columns or rows is not
allowed in the first place and resulted in surprising user-visible
behaviour: e.g. when using row selection, clicking mostly anywhere in
the row selected it, except if the click happened to be between the two
columns, in which case it didn't.

Fix this and always select the row in such scenario now.

Unfortunately, doing this required adding yet more CanDragXXX()
functions in addition to the already impressive panoply of them in
wxGrid, but we need CanDragGridColEdges() as none of the existing
functions checked for m_useNativeHeader (there was instead an ad hoc
check for it directly in the mouse handling code) and the row version
had to be added for symmetry.

interface/wx/grid.h

@@ -4322,6 +4322,26 @@ public:
     */
     bool CanDragColSize(int col) const;
 
+    /**
+        Return @true if column edges inside the grid can be dragged to resize
+        the rows.
+
+        @see CanDragGridSize(), CanDragColSize()
+
+        @since 3.1.4
+     */
+    bool CanDragGridColEdges() const;
+
+    /**
+        Return @true if row edges inside the grid can be dragged to resize the
+        rows.
+
+        @see CanDragGridSize(), CanDragRowSize()
+
+        @since 3.1.4
+     */
+    bool CanDragGridRowEdges() const;
+
     /**
         Return @true if the dragging of grid lines to resize rows and columns
         is enabled or @false otherwise.