Initial CMake build system implementation

[YakoYakoYokuYoku]

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• 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

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

• New feature (non-breaking change which adds functionality)

What does this pull request do?

Due to some limitations of the QMake build system, like easily customizing targets for example, I've decided to add a port for CMake alongside the existing QMake. This makes some tasks like doing Shiboken generation or the generation of an Engine Python module much more easier to implement and it let us use Ninja too.

Show a few screenshots (if this is a visual change)

N/A.

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

By building and running Natron, NatronRenderer, natron-python and the test suite.

Futher details of this pull request

Untested in OSX and Windows. Qt4 is not supported because its absense in many upstream package distributions.

[devernay]

why were RotoGui.cpp and RotoGui.h removed?

[YakoYakoYokuYoku]

Those two files are in fact actually not used anywhere, they are not in Gui.pro and they were deleted at some point (can't get the sig of the commit, I lost track of it the hash is a11e076).

[devernay]

these are non-breaking changes (my only concern was the removal of RotoGui.*)