Refactor export functions

Some code refactoring for the various export functions:

1)
I separated out the new function panels_pane_export() from panels_export_display() for some reasons:
- A pane is logically an object on its own, and not part of the display object, so it deserves its own export function, even if we have no UI for exporting panes.
- The code gets easier to grasp and to manage.
- This is a step towards OOP.
- It doesn't break anything.
I called the function panels_pane_export (as panels_var_export) and not panels_export_pane (as panels_export_display), as the former naming is more OO.

2)
I included $display->did, $panel_page->pid and $panel_mini->pid back into the foreach construct. It's simply cleaner to set them in the object and then export them as any other fields.
To make this possible, I had to add the 'pid' field to panels_page_fields() and unset it in panels_page_save(), which is basically a good thing. panels_page_fields is basically a poorman's SchemaAPI, and there is no reason to leave a field out there.

3)
In panels_page_export, I unset $fields['displays'] before entering the foreach construct, instead of doing exception handling within the foreach construct. It's just a bit cleaner.

4)
I corrected the not really consistent indentation, so the output is better structured now.

5)
panels_var_export() used to do the str_replace($output) even if $output = 'array()'. This was unnecessary and bloated the code.

This will break #260278 vice versa, so after you committed either of them, I need to reroll the other.