WIP: add a fixed autodj tab to the library

[nopeppermint]

on top of #1671 I like to add a optional, fixed autodj tab to the library, as shown here with latenight

this autodj tab on the right will be fixed on autodj, therefore no controll buttons are required for this tab, just one button on the toolbar to enable/disable this option

This would be a really useful tab for me, as at the moment I am switching constantly from autodj view to tracks/playlist or crates view..

[nopeppermint]

at the moment Autodj is fixed to this special tab and can not be used on the first tab, even if the second is not displayed... so this is not the final solution, except this is a new feature and autodj could be only used in this new tab from now on ;-)

[Be-ing]

Let's work on flexible, robust designs instead of hacks that only work for one specific use case.

[ronso0]

@nopeppermint
I remember a very long discussion about how to design the multi-pane library, and we worked towards a solution that fits more than one use case. If you're impatient to wait for this, just built a private branch ;)
Please refer to the first (or second?) lib redesign PR. I don't find it right now, but starting at the middle part of the discussion, there were a lot of illustrated ideas how to deal with multiple button bars and so on.

[ronso0]

I don't find it right now,

Got it, it's actually #1117 But that chat is looong so you'd have to expand hundreds of hidden items.
Have a look at around July 2017

[nopeppermint]

ok