Fix tags issue while alternating visual and text mode in the classic editor

[renatho]

Fixes #128 and fixes #127

Changes proposed in this Pull Request:

• Fix some issues while alternating between visual and text mode in the classic editor. The main issue is that it was creating some new lines.
• The other issue mentioned is that the angle brackets were appearing as <. I could not reproduce it, but I believe these changes will fix it as well.

The main idea of this solution was to restore the unfiltered codes after the afterWpautop.

• I also identified that <p> and <br> tags were being removed in the core. So I replaced it temporarily to <wp-p> and <wp-br> tags. It'll appear only in the visual mode if the user changes from the text mode.

Testing instructions:

• Install and active the classic editor.
• Create a new post with the SyntaxHighlighter shortcode [code language="php"] or [html]. Please, test mixing and not mixing other texts in the post.
• Also test using <p> and <br /> tags inner your code.
• Make sure that the shortcodes are keeping the indentation and break lines correctly, as well the angle brackets. And the rest of the content continues being treated as before.
• If possible, also test using the WP 4.2.3 (where the errors happened too) to confirm everything is working well. I tested here and it seems to be working fine as well.

[jom]

I did some testing on this today. When I switched back to Visual the tags were replaced, is that expected? It looks like it didn't save with them though.

[renatho]

I did some testing on this today. When I switched back to Visual the tags were replaced, is that expected? It looks like it didn't save with them though.

@jom, yes. It's expected because of that:

I also identified that

and
tags were being removed in the core. So I replaced it temporarily to and tags. It'll appear only in the visual mode if the user changes from the text mode.

Unfortunately, I didn't find a way to prevent it from being removed unless to replace it with a temporary mark.

[jom]

Ahh, sorry I missed this. It works well! Nice reorganization, too.