Restore some whitespace to the mixer channel layout

[sakertooth]

Trying to reach a compromise with #7505 and #7502.

I think I initially believed that users were complaining about the spacing being too much horizontally. However, the vertical spacing in QVBoxLayout was 6 for me by default, which could be a bit large, and wasn't set to something smaller in #6591.

I changed the margin size and spacing to make the mixer channel a bit shorter, while also trying to not compromise useful whitespace.

I'm waiting for two approvals for this PR in particular before merge (I should probably require two approvals in general anyways).

Fixes #7505 (I think)

[michaelgregorius]

Look of the mixer with the changes in this PR as of now:

[sakertooth]

Thoughts @michaelgregorius?

[sakertooth]

Here's an updated image for how the mixer channel's look now:

I am very satisfied with this, but I'm waiting for other people's opinions before merge.

[Rossmaxx]

Looks good.

[michaelgregorius]

I am very satisfied with this, but I'm waiting for other people's opinions before merge.

If this was mainly about the horizontal space then I'd appreciate if there was a bit more margin at the bottom where the fader ends.

Otherwise it looks okay to me.

[sakertooth]

What do we think about moving the peak indicator to the bottom? It adds some space for the fader at the bottom, and I think looks better? I probably won't merge this in (I'm foreshadowing backlash), but just a thought.

[Rossmaxx]

Peak indicator at bottom doesn't look good according to me. It seems to increase the view angle, but that might be just me. It's something i cam get used to so it's not really a blocker.

[sakertooth]

Peak indicator at bottom doesn't look good according to me. It seems to increase the view angle, but that might be just me. It's something i cam get used to so it's not really a blocker.

Yeah, I'll leave it for now but just add some more whitespace at the bottom for the fader.

[sakertooth]

How it looks as of the time of merge: