Library track selection tweaks

[ronso0]

follow-up of #4177, picking bites from #3063 to improve the track selection when changing features and search queries.

• only attempt to load a saved selection state if the new search query is more specific

[ronso0]

Note: I marked it as draft because I'll likely rebase -- still want to test and work out some details.
Thanks for the early review though!

[ronso0]

Actually, this is it.
initially I was planning to pick save TrackIds instead of index if possible from 7b9e405, too, but this state here is sufficient for now, so I'll resume some other PRs instead.

[poelzi]

Saving only indexes is super annoying, thats why I implemented the IDs. I often use a filter like
"genre played:0" and play some stuff. If tracks are playing and you are in the history for example and switch back, causes the wrong selection. So, the new code tries to save trackids if the view supports those.

[ronso0]

Sure. you can open a PR at my branch with that change (only) that also saves the focused column of the current TrackId's row because that's what I missed last time I tested it.

Sooner or later I'll do it myself, but not in this PR

[ronso0]

Okay, now this works also if an invalid (invisible) column is passed to setCurrentTrack() in case it is invoked from somewhere else then onSearch().

[ronso0]

@uklotzde
I will squash the last --fixup commit after the final LGTM

[uklotzde]

I cannot promise that there are no regression. But code looks good, so let's move on. Thank you! LGTM

[uklotzde]

However the local build with Qt 5.15 failed:

/home/uk/volumes/Build/cpp/mixxx/src/library/searchqueryparser.cpp:396:77: error: 'SkipEmptyParts' is deprecated [-Werror,-Wdeprecated-declarations]
    QStringList queryWordList = query.split(kSplitIntoWordsRegexp, QString::SkipEmptyParts);
                                                                            ^
/usr/include/qt5/QtCore/qstring.h:605:24: note: 'SkipEmptyParts' has been explicitly marked deprecated here
        SkipEmptyParts Q_DECL_ENUMERATOR_DEPRECATED
                       ^
/usr/include/qt5/QtCore/qcompilerdetection.h:1160:40: note: expanded from macro 'Q_DECL_ENUMERATOR_DEPRECATED'
#  define Q_DECL_ENUMERATOR_DEPRECATED Q_DECL_DEPRECATED
                                       ^
/usr/include/qt5/QtCore/qcompilerdetection.h:227:45: note: expanded from macro 'Q_DECL_DEPRECATED'
#  define Q_DECL_DEPRECATED __attribute__ ((__deprecated__))
                                            ^
/home/uk/volumes/Build/cpp/mixxx/src/library/searchqueryparser.cpp:396:39: error: 'split' is deprecated: Use Qt::SplitBehavior variant instead [-Werror,-Wdeprecated-declarations]
    QStringList queryWordList = query.split(kSplitIntoWordsRegexp, QString::SkipEmptyParts);
                                      ^
/usr/include/qt5/QtCore/qstring.h:627:23: note: 'split' has been explicitly marked deprecated here
    Q_REQUIRED_RESULT QT_DEPRECATED_VERSION_X_5_15("Use Qt::SplitBehavior variant instead")
                      ^
/usr/include/qt5/QtCore/qglobal.h:374:45: note: expanded from macro 'QT_DEPRECATED_VERSION_X_5_15'
# define QT_DEPRECATED_VERSION_X_5_15(text) QT_DEPRECATED_X(text)
                                            ^
/usr/include/qt5/QtCore/qglobal.h:294:33: note: expanded from macro 'QT_DEPRECATED_X'
#  define QT_DEPRECATED_X(text) Q_DECL_DEPRECATED_X(text)
                                ^
/usr/include/qt5/QtCore/qcompilerdetection.h:675:55: note: expanded from macro 'Q_DECL_DEPRECATED_X'
#    define Q_DECL_DEPRECATED_X(text) __attribute__ ((__deprecated__(text)))
                                                      ^
2 errors generated.

Fixed by #4609.