US39: Add a logging facility

- Add Log class, to format log messages with different priorities.
- Add command line options to select verbose level.
- Use Log class to output debug messages from MetaData.
Fix stupid for loop
US28 Start creating the IFD structures
US28: New DngFloatWriter class

- Move new code to a different class, DngFloatWriter.
- Add support to write the main IFD with some example tags.
US28: Add raw data to the new writer.

- The new writer writes all the required tags and the raw data. No previews yet.
- Fixed orientation value in MetaData.
US28: Separate TIFF structures from writer.

- Put class IFD in its own file.
- Add progress advances to DngFloatWriter::write
US28: Get rid of third party libs

- Remove the dependency on DNG and XMP SDK from CMakeLists.txt. Also, look for ZLib.
- Add method to copy metadata from the least exposed image to the output file with Exiv2.
- Add the standard TIFF/EP tags to the output file.
US28: Move tone curve to ImageStack
US28: Use DngFloatWriter in ImageStack
US28 Fix gcc warning
US28: Add preview renderer.

- Add Renderer class to (fast) interpolate the raw data and create a preview of the image.
- Use Renderer in DngFloatWriter to create the thumbnail and the preview.
US28: Correctly copy most metadata.

- Fix orientation tag with correct TIFF values.
- Fix Make and Model tags with information from source images. LibRaw provides other names.
- With correct Make tag, makernotes are correctly copied.
US28: Remove the remaining files from DNG SDK

- Remove all the third party files, exiv2meta and DngWriter classes.
- Remove XMP SDK compilation instructions from README.
Fix highlight clipping in previews
US32: Make DngPropertiesDialog subclass SaveOptions
US32: Add a load dialog.

- Add LoadOptionsDialog, with a list of files where the user can add and remove images, and a checkbox to align them.
- Move all the "open files" functionality from MainWindow to LoadOptionsDialog.
US32: Translate texts
US38: Move edit actions to EditableMask.

- Renamed MergeMap as EditableMask. Renamed anything called *index* to *mask*.
- Move DngFloatWriter::buildIndexImage to EditableMask::writeMaskImage.
- Move EditAction, undo and redo from PreviewWidget to EditableMask.
- Fix Bitmap::applyRowMask, with a buffer overflow under certain situations.
- Fix Image::relativeExposure to use all the common area between two images.
- Fix ImageStack::align to align images from least to most exposed.
- Renamed ImageStack::findIntersetion as ImageStack::crop, because it crops the image to the optimal size.
  Otherwise, the active area is the least exposed image.
- Add checkbox to LoadOptionsDialog to crop the final image.
- Fix Render, to clip negative values.
US38: Small optimizations.
US38: Optimize mask creation.

- Optimize the mask creation process to make it faster.
- Fix Bitmap::resize to reset the last unused bits.
- Add a saturation threshold 1% lower than max value.
- Add time measurement methods to Log.hpp
- Fix tests that have changed since last commit.