Allow for more customization of show action panels 

Had a recent need to customize show action for AA resource. I like how there is active_admin_comments helper, which allows to display the admin comments panel. And then there is default_main_content, which displays both the resource properties **AND** the comment form. 

But what if one wished to place a panel between the default panels? Well, I know you can use 
```attributes_table :attr_a, :attr_b, :etc``` 
OR
```attributes_table *default_attribute_table_rows```

But why dive into internals or repeat oneself if the default way to list the attributes is good enough? Right?

With these two commits, you can now render main show action panel separately.

```ruby
show  do
  default_main_content
  panel "X" do
    ...
  end
  active_admin_comments
end
```

You can still get the default way of doing things:
```ruby
show do
  default_main_content_with_admin_comments  # too verbose?
end
```
Note on compatibility:
Unlike its sibling **active_admin_comments**, **default_main_content** method isn't publicly documented. This change would only affect those who made an effort to dig into AA internals. Any existing project explicitly calling **default_main_content** while customizing show action will lose the active_admin_comments panel (if it was enabled). But the same functionality can be restored by calling **default_main_content_with_admin_comments** now instead. Not a big deal really.