Remove meta class from HUD

[pixelmachine]

At the moment the HUD uses the 'meta' styling for the fields. Whilst this make sense in theory (it's narrow, the meta box is narrow) in reality it results in a fairly messy layout with fields much narrower than they need to be.

With a just few minor changes the usability can be much improved:

1. Remove the .meta class
2. Add a little padding

.hud .elementeditor .main { 
  padding: 0 24px 24px 
}
.preview-thumb-container.editable {
  margin-bottom: 24px;
}

Adjusting the width to be proportional with a larger maximum (if needed at all) would solve a couple of other open issues:

.hud .elementeditor {
  width: 50vw;
  min-width: 320px;
  max-width: 800px; // Or no max?
}

Before

After

Would solve #4129 I think.
Solves #5948 if editing via the HUD but not asset entry.

I'm not sure if there any other contexts of the HUD that I haven't considered.

[brandonkelly]

This is resolved for 3.7! See #8017.