Return empty bitmaps from wxButton if not explicitly set in wxMSW

Do this for consistency with the other ports and because this seems more
useful anyhow.

Update the documentation to make this behaviour more clear and document
this change as a (minor) incompatibility in wxMSW.

Also add more unit tests to check for this behaviour. Note this also
fixes the problem with the unit test added in the grandparent commit
under MSW.

interface/wx/anybutton.h

@@ -41,10 +41,10 @@ public:
     wxBitmap GetBitmap() const;
 
     /**
-        Returns the bitmap used when the mouse is over the button, which may be
-        invalid.
+        Returns the bitmap used when the mouse is over the button.
 
-        @see SetBitmapCurrent()
+        The returned bitmap is only valid if SetBitmapCurrent() had been
+        previously called.
 
         @since 2.9.1 (available as wxBitmapButton::GetBitmapHover() in previous
             versions)
@@ -52,18 +52,20 @@ public:
     wxBitmap GetBitmapCurrent() const;
 
     /**
-        Returns the bitmap for the disabled state, which may be invalid.
+        Returns the bitmap used for the disabled state.
 
-        @see SetBitmapDisabled()
+        The returned bitmap is only valid if SetBitmapDisabled() had been
+        previously called.
 
         @since 2.9.1 (available in wxBitmapButton only in previous versions)
     */
     wxBitmap GetBitmapDisabled() const;
 
     /**
-        Returns the bitmap for the focused state, which may be invalid.
+        Returns the bitmap used for the focused state.
 
-        @see SetBitmapFocus()
+        The returned bitmap is only valid if SetBitmapFocus() had been
+        previously called.
 
         @since 2.9.1 (available in wxBitmapButton only in previous versions)
     */
@@ -82,9 +84,10 @@ public:
     wxBitmap GetBitmapLabel() const;
 
     /**
-        Returns the bitmap for the pressed state, which may be invalid.
+        Returns the bitmap used when the button is pressed.
 
-        @see SetBitmapPressed()
+        The returned bitmap is only valid if SetBitmapPressed() had been
+        previously called.
 
         @since 2.9.1 (available as wxBitmapButton::GetBitmapSelected() in
             previous versions)