Follow-up: Library prefs

[uklotzde]

Some trivial changes and cleanup.

Unfortunately, the "[Library]" config group is used for all kinds of settings, not only those that belong to the Library class.

[coveralls]

Pull Request Test Coverage Report for Build 1154262831

• 11 of 16 (68.75%) changed or added relevant lines in 6 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.005%) to 25.975%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dialog/dlgkeywheel.cpp	0	1	0.0%
src/track/track.cpp	0	1	0.0%
src/widget/wtracktableview.cpp	4	5	80.0%
src/library/library.cpp	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/engine/cachingreader/cachingreaderworker.cpp	2	63.48%

Totals
Change from base Build 1153454756:	0.005%
Covered Lines:	20016
Relevant Lines:	77060

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💛 - Coveralls

[daschuer]

LGTM, Thank you.

[uklotzde]

To be discussed: I noticed that ConfigKey is used for addressing both settings and control objects. In this respect we might decide to rename the nested prefs namespace to ctrl or whatever is more suitable to cover all use cases.