US31: Move the image index to MergeMap
US31: Increase borders of masked pixels

- Leave basic saturation check in Image, move inteligence to MergeMap.
- Erase the mask around saturated pixels to avoid merging these pixels in the output image.
US30: Add preview width and index file name props

- Add a preview size selector to DngPropertiesDialog.
- Add an index image file name selector to DngPropertiesDialog.
- Add a cancel button to that dialog.
- Make MainWindow set the selected properties on DngWriter.
US30: Add two additional properties to the DNG

- Change selectors to radio groups in DngPropertiesDialog.
- Add a selector for the preview width.
- Add a textfield for the mask file name.
- Add command line options for these two properties.
- Add translations.
Fix incorrectly scaled previews
Add three more colors to the preview, up to 7
Improved MTB algorithm.

- Calculate mtb and exclusion bitmaps at the prescale stage. Full scaled images can be then released.
- Reduce the memory consumption by releasing the bitmaps after the alignment.
Use different threshold to align each image.

- Go back to the previous version of the align algorithm, so that it can use a different
threshold to align each pair of images.
- Calculate the alignment threshold discarding saturated pixels. Using always the median
fails with too bright images.
Minor improvements in GUI

- Do not save the index image by default. Add a checkbox to enable/disable it,
  instead of setting an empty file name.
- Reset the default tool when a dialog is open, to avoid it getting stuck.
- Add and fix spanish translations.
- Add explanations on the usage to the README, and the acknowledgments.