Shiboken2 postmerge fixes in Engine

[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)

• 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?

This is a series of fixes after #697 was committed.
After the merge the enums of the Engine typesystem were placed in NatronEngine instead of NatronEngine.Natron, change which breaks scripts that used said enums. This PR amends the regression.
This change also makes the hidden set methods in Int2DParam and Int3DParam private so they are not called where it's not supposed to be.
And lastly the Engine.pro was modified to build with the Shiboken2 binding, which they'll reside in Engine/NatronEngine5.

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

N/A.

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

Started Natron with this init.py to assert that the enum was in the correct place.

import NatronEngine

print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsRGB)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsR)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsG)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsB)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsA)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsY)
print(NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsMatte)

NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsR
NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsG
NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsB
NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsA
NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsY
NatronEngine.Natron.DisplayChannelsEnum.eDisplayChannelsMatte

Tried with natron-python but the thing turned out to be disobedient.

Futher details of this pull request

N/A.

[devernay]

thanks