Simplify API for extending wxListCtrl background display

Replace SetListRulesAlternateColourOnBlank() taking 2 arguments, with
the second of them being used only when the first one is true, with a
simpler but still sufficient ExtendRulesAndAlternateColour(bool).

Make the new method virtual and define it as doing nothing in
wxListCtrlBase class, so that it's still available, even if currently
not implemented, in wxMSW.

Also simplify the implementation, fix style problems and other minor
improvements.

interface/wx/listctrl.h

@@ -1334,28 +1334,31 @@ public:
     void CheckItem(long item, bool check);
 
     /**
-        When the content of state is true, tells listctrl to draw
-        horizontal and vertical rulers on blank rows and also to
-        paint the background of alternate rows on blank rows, using
-        colour to paint the alternate blank(empty) rows' background.
+        Extend rules and alternate rows background to the entire client area.
 
-        As EnableAlternateRowColours(), this method can only be used with
-        controls having ::wxLC_REPORT and ::wxLC_VIRTUAL styles.
+        Bu default, the rules (when enabled with wxLC_HRULES and wxLC_VRULES)
+        and alternate row background (when EnableAlternateRowColours() was
+        called) are only shown in the part of the control occupied by the
+        items, which can be smaller than the entire window if there are few
+        items in the control.
 
-        This method is NOT available on Windows.
+        Calling this function extends the display of the rules and alternate
+        background rows to the entire client area.
 
-        @param state
+        Similarly to EnableAlternateRowColours(), this method can only be used
+        with controls having ::wxLC_REPORT and ::wxLC_VIRTUAL styles.
+
+        Note that this method is currently not implemented in the native MSW
+        version and does nothing there.
+
+        @param extend
             if @true, draws horizontal rules and vertical rules on empty rows
             and uses the colour parameter to paint the background of
             alternate rows when those rows are blank, empty, with no data.
-        @param colour
-            A valid row background colour to enable alternating rows on
-            blank rows (rows with no data).
 
         @since 3.1.5
-        @note This method is currently NOT implemented in the Windows version.
     */
-    void SetListRulesAlternateColourOnBlank(const bool state, const wxColour& colour);
+    void ExtendRulesAndAlternateColour(bool extend = true);
 
 protected: