PCM: add ability to pin packages

Pinned packages don't affect available update notification and will not
be updated with "Update All" button. Manual update is still possible but
will trigger a confirmation dialog.

common/widgets/split_button.cpp

@@ -88,6 +88,13 @@ void SPLIT_BUTTON::SetBitmap( const wxBitmap& aBmp )
 }
 
 
+void SPLIT_BUTTON::SetLabel( const wxString& aLabel )
+{
+    m_label = aLabel;
+    Refresh();
+}
+
+
 wxMenu* SPLIT_BUTTON::GetSplitButtonMenu()
 {
     return m_pMenu;