Updated screenshots for wp.org repo

[netweb]

I updated bbPress' screenshots for the wp.org repo

​https://plugins.trac.wordpress.org/browser/bbpress/assets

Screenshots

1. Forums - Admin Interface (screenshot-1-new.png)
2. Topics - Admin Interface (screenshot-2-new.png)
3. Replies - Admin Interface (screenshot-3-new.png)
4a. Settings - Admin Interface (screenshot-4-new.png)
4b. Settings - Admin Interface (Alt full list of settings screenshot) (screenshot-4-new-alt.png)
5. Mint - Admin Theme ( screenshot-5-new.png)
6. Evergreen - Admin Theme (screenshot-6-new.png)
7. Single Forum - Default Theme (screenshot-7-new.png)

Code for readme.txt for /runk and 2.5 branch

== Screenshots ==
1. Forums - Admin Interface
2. Topics - Admin Interface
3. Replies - Admin Interface
4. Settings - Admin Interface
5. Settings 2 - Admin Interface
6. Themes - Admin Interface
7. Single Forum - Default Theme

Should we do screens for bbPress Tools? (Repair Forums, Import Forums & Reset Forums)

Which screenshot for 4.? 4a (screenshot-4-new.png) or 4b (screenshot-4-new-alt.png) ???

[johnjamesjacoby]

In 5318:

Add screenshots to readme.txt. Props netweb. See #2555.

[netweb]

In WordPress Plugins ​r872931: bbPress: Update screenshots.

[johnjamesjacoby]

Assigning all closed & unassigned tickets in the 2.6 milestone to myself.