Playlists & Crates: Store selected TrackId instead of TrackPointer

[uklotzde]

...to avoid delays when exporting file tags.

Another low-hanging fruit. See also: https://mixxx.discourse.group/t/tag-export-question-2-3-0/22822

[coveralls]

Pull Request Test Coverage Report for Build 1121569811

• 0 of 21 (0.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.004%) to 26.004%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/library/trackset/baseplaylistfeature.cpp	0	8	0.0%
src/library/trackset/crate/cratefeature.cpp	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/library/trackset/baseplaylistfeature.cpp	1	0%
src/library/trackset/crate/cratefeature.cpp	1	0%
src/engine/cachingreader/cachingreaderworker.cpp	2	63.48%
src/engine/readaheadmanager.cpp	2	88.89%

Totals
Change from base Build 1117978159:	-0.004%
Covered Lines:	20021
Relevant Lines:	76991

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💛 - Coveralls

[daschuer]

LEGT, thank you.