Disable focus for new channel button

[sakertooth]

Disable grabbing focus for the new channel button in Mixer. I think this button grabbing focus goes against expectations but let me know what you think.

[michaelgregorius]

I guess it makes sense as there's also no indication that the button has the focus.

For anybody wondering, the following does not work anymore after this PR:

• Click the "Add channel" button
• Hit the space bar several times to add more channels

Does not seem critical to me though because I assume that it's not really a common use case that users want to add tons of channels upfront and quickly as described above.

[sakertooth]

I made this PR mainly because users may want to start and stop the song with the Spacebar, but they might not even realize that the new channel button has focus. If any major objections pop up though, I am mostly fine with reverting.