changed: rename PLAYLIST namespace to KODI::PLAYLIST

[notspiff]

Description

Move PLAYLIST namespace under KODI namespace

Motivation and context

More consistent with game, video, ..

How has this been tested?

It builds.

What is the effect on users?

None

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[fuzzard]

Had a scroll through, looked ok at a glance.

[garbear]

Nice! I like seeing namespaces predating KODI moved back inside it.