Color selector palette

[rodlie]

• [x] I've followed the contributing guidelines to the best of my understanding
• [x] I've read and understood the contributing guidelines
• [x] I've formatted my code according to Natron's code style
• [x] 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)
• [x] 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?

Adds supports for palette in the color selector and some ui changes/fixes in the widget.

The palette is stored in the Natron settings.

• Right-click a palette color to clear/reset
• Shift+A to add current color to palette
• Shift+C to clear the palette

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

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

• macOS 10.13
• Ubuntu 20.04
• Windows 10 20H2

Futher details of this pull request

See #688

[devernay]

@YakoYakoYokuYoku I merged this into RB-2.5 but didn't check that it doesn't break the Qt5 build

[YakoYakoYokuYoku]

Everything went well in the build without any issue