make export/import output valid xml and parse xml using simplexml

[tott]

WXR Export as well as export.php script is not properly indented.

Attached is a small fix that produces well indented xml and improves readability in export.php script.

patch against rev 10020 indenting wxr exports as well as improving readbility in export.php

The phpdoc should not be indented. It is good as it is.

Based off of r10258

• Separates functions out of the export_wp() function.
• indents the lines inside export_wp().

The problem is that since the import uses regular expressions, you can't really change the indentation without correcting the importer as well. When the importer switches over to using XML, then it can be done.

-1 for SimpleXML, +1 for DOM-XML. Just to save us some headaches then.

see also #9633.

Equally see #7400.

Maybe even drop DOM-XML because SimplePie uses XML Parser Functions: ​http://us2.php.net/manual/en/ref.xml.php

Those are PHP4 as well.

(In [15961]) Importer and exporter overhaul, mega props duck.

Exporter overhaul:

• Add author information to export
• Greater usage of slug identifiers
• Don't export auto-drafts, spam comments, or edit lock/last meta keys
• Inline documentation improvements
• Remove filtering for now (@todo)
• Bump WXR version to 1.1, but remain back compat in the importer

Importer overhaul (​http://plugins.trac.wordpress.org/changeset/304249):

• Use an XML parser where available (SimpleXML, XML Parser)
• Proper import support for navigation menus
• Many bug fixes, specifically improvements to category and custom taxonomy handling
• Better author/user mapping

Fixes #5447 #5460 #7400 #7973 #8471 #9237 #10319 #11118 #11144 #11354 #11574 #12685 #13364 #13394 #13453 #13454 #13627 #14306 #14442 #14524 #14750 #15055 #15091 #15108.

See #15197.