Modify help tab text for new Themes page (post-THX)

[Hanni]

The help tab text on themes.php needs to be updated to reflect the new themes.php layout.

Here's a very rough first pass, help please! :)

Looking at the text it also occurred to me that the sentence "Aside from the default theme included..." isn't quite correct, since previous years' themes are included too. Made a small yet possibly unsatisfactory edit there.

[designsimply]

I patched this a few weeks ago in the THX38 plugin at ​http://plugins.trac.wordpress.org/changeset/802724/thx38 but it looks it didn't get merged in yet.

[designsimply]

26285.1.diff moves the plugin patch to a core patch. The coding follows the formatting used in nav-menus.php

26285.2.diff is just text updates if you'd prefer not to change the surrounding code.

Both patches address the "Aside from the default theme(s) included" text Hanni mentioned.

[dd32]

In 26517:

Themes: Add a updated Help text. Props designsimply & hanni. See #26285

[dd32]

I altered a little bit of text, take a read through and see if anything jumps out as needing changing.
I'll leave it up to you to notice the exact changes made.

[ethitter]

Update text about adding themes

[ethitter]

26285.patch​ updates the Adding Themes tab to reflect that the Install Themes tab is now an Add New button.

[nacin]

In 26759:

Update the help text on themes.php. The Install Themes tab is now an Add New button.

props ethitter.
fixes #26285.