Added random sort "easter egg" and added tooltip for the random sort

[jmigual]

Hello,
I have added thhe random sort discussed in bug #1447082

[daschuer]

Thank you for the PR. It works :-)
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and comment here when done.

[daschuer]

Use
fieldIndex(ColumnCache::COLUMN_LIBRARYTABLE_PREVIEW)
to make the code robust against future changes.

[daschuer]

Some extra findings, but nothing that stops us from merging:

Playing with it the sorting arrow on the preview header feels misplaces since random has no reverse.

It would be cool to have a the soring hierarchy working with random.
Use case. sort random and than sort by star rating. Result: random soring of all 5 star tracks.

[jmigual]

Ok, now I've made the changes and signed to become a contributor :)

[daschuer]

Thank you. Are you working on the hierarchical random sort or should we merge this as it is?

[jmigual]

I think we would better merge as it is, but we can add it to the wishlist.