Random sorting is not persistant

Now Mixxx supports random sorting by clicking on the preview header.

Unfortunately, we will get a new random sorting every time we access the database. This makes the Random result volatile, which seams to be not what the user expect.

In the right click test case, it looks like there is a redundant DB lookup, which can be probably removed.

To solve the issue entirely, we may add a new hidden column or a temp table to the database that holds the "last_random_value". Every random sorting should use BY last_random_value. Then we need a new "Randomize" query, which refreshed the last_random_value column on every header toggle from ASC to DESC.