Implement protectTags functionality in package export and re-import

The protectTags feature is not reflected in package export and re-import.

Normal Im- and Export:

With protectTags enabled, the following content is importable in translate5:{}

<source>&lt;b&gt;Test&lt;/b&gt;</source>

will be imported to:

<internaltag for b open>Test<internaltag for b close>

And on export the content looks again like:

<target>&lt;b&gt;Test&lt;/b&gt;</target>

Needed changes for package export and re-import:

• On package export the internal tag should be exported as bpt/ept/ph tag containing the encoded original tag - currently this is not the case and the so exported XLF is not really usable in third party software
• Distinguish with export context (editor_Models_Export::EXPORT_PACKAGE)
• Export should look like (valid btp/ept tags with attributes etc)

<target><bpt>&lt;b&gt;</bpt>Test<ept>&lt;/b&gt;</ept></target> 

• Re-Import should handle the tag, so that it finally the original tag is stored (distinguishable since protectTags in task config is true), in other words only the content between bpt and ept should be used, not the bpt / ept itself.