AutoDJ: make 'enable' shortcut work after startup

[ronso0]

I noticed two issue I need help with.
and I rather discuss them here to work around launchpad and ease code references.

1) before the AutoDJ hotkey Shift+F12 would toggle [AutoDJ],enable after startup any library feature must have had focus (Ctrl+F to focus the searchbar is sufficient, just ANY trackmodel needs to be selected?)

2) with 'Allow random track addition' disabled the above wouldn't work either:

• add some tracks to the AutoDJ queue, close Mixxx
• start Mixxx
• Ctrl+F to focus the searchbar
• press Shift + F12 to start AutoDJ
  = log "Debug[Main] Queue is empty now"
  from https://github.com/mixxxdj/mixxx/blob/2.3/src/library/autodj/autodjprocessor.cpp#L403
  because getNextTrackFromQueue() returns 0 even though the queue holds enough tracks.
• click AutoDJ feature
• press Shift + F12 to start AutoDJ
  = AutoDJ starts

2 is solved with change added here: m_pAutoDJTableModel->select(); in autodjprocessor.
Though this is just a hack, I have no idea about the implications.

For 1 I have no idea..

[daschuer]

A controller that has mapped [AutoDJ],enabled is also affected.

I think that it is sufficient that m_pAutoDJTableModel->select(); is called once.

[ronso0]

what about

1. before the AutoDJ hotkey Shift+F12 would toggle [AutoDJ],enable after startup any library feature must have had focus

I doubt the change here fixes it for controllers as the shortcut is not working.
Where else do we need to m_pAutoDJTableModel->select(); ?

[ronso0]

@daschuer Works beautifully, ready for review!

[daschuer]

LGTM, Thank you for taking care.