Improved search and import mode writing output pack files to different folder.
Modified search and import mode to keep the original palette
Fixed several bugs about fixed palettes

* FixedPalette.cs: Added indirect conversion for some color spaces
  (ie: RGBA -> RGB -> Lab)

* Pack.cs: Fixed bug trying to close null subfiles

* Importer.cs: Fixed condition of FormatException about max colors,
  the equals must not be included
Fixed out of range exceptions in dithering method

* Program.cs: Fixed problem with pack files without palette