Save [Options]/CheckedBgZoom as bool value
Fix a bug with invalid m_ptr in image iterators when we merge down layers (fix issue 358)

The ImageIteratorT implementation was simplified to avoid invalid m_ptrs.
Fix bug in ColorPicker::pickColor() getting colors outside the image bounds
Fix issue 360: Cannot save gif files

* SaveFile and SaveFileAs commands ask for removing read-only attribute
* Fixed base::open_file_descriptor_with_exception() to create new files
* Added internal base::Win32Exception
* Added unit tests for the creation of file descriptors
* Added base::delete_file, has_readonly_attr, remove_readonly_attr functions