Improve project loading/saving time by using binary format

[cedricp]

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

Add the possibility to load/save project in binary format to speed it up. Some of my projects could take several seconds to serialize and deserialize. Now the project is saved and restored almost immediately.
The XML format is still the default choice, but the preference panel gives the ability to switch.

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Saving a project was way too long, and the autosave feature slows down your workflow

Have you tested your changes (if applicable)? If so, how?

Several tests with and without.