Do not accept loading tracks if pass through is enabled on a deck

[jus]

Steps to reproduce:

* Activate pass through on a deck, see https://bugs.launchpad.net/mixxx/+bug/355945/comments/11
* If pass trough is active load a track to the deck ( via controller oder mouse)

Actual result:
* Track loads, but no waveform is drawn and you can not start playback. This is just wrong, as you be able to use Mixxx EQ`s and Mixer on the live input signal , but the Deck show a complete different track.

Expected result:
*Loading track should be rejected, under ideal circumstances a note might be put out to the waveform that passthrough is enabled. We currently lack GUI feedback for pass trough at all. This is a separate bug.

We might also handle the corner case where a track is already loaded when pass through is enabled on the deck.

[Owen Williams (ywwg)]

I wonder how hard it would be to write a waveform widget that is generated from the live audio data? so it would look like a regular track only there wouldn't be anything to the right of the playhead.

[jus (jus)]

Seealso lp:1269148

[RJ Skerry-Ryan (rryan)]

Based on discussion on Bug #1269148 it sounds like we want the deck to function normally but just show an overlay image on the waveform summary / overview. So should this bug be WontFix?

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/7187