Dashicons: Include the easy-access classes for plugins

[dd32]

Currently the version of Dashicons in core doesn't include the easy-access/easy-use CSS classes such as .dashicons and .dashicon-wordpress, which forces themes to use the glyphs directly by specifying the fonts and content:'\f155';.

Instead, we should include the full set of icon classes as visible within the Dashicons CSS: ​https://github.com/melchoyce/dashicons/blob/master/css/dashicons.css

[melchoyce]

I kind of guessed on some of the class names — I'd appreciate a second look to make sure everything I've named them makes sense.

[dd32]

26227.diff​ is the extra rules.

The class names are visible within there, and also on the demo site here: ​http://melchoyce.github.io/dashicons/

I personally haven't seen any I disagree with, but it'd be helpful if others could comb through it as well.

[nacin]

In 26547:

Add dashicons CSS classes for easy access by plugins and themes.

props dd32.
fixes #26227.

[melchoyce]

In 26227.2.diff​, I updated the following icon class names:

• dashicons-admin-site --> dashicons-admin-network
• dashicons-site --> dashicons-admin-site
• dashicons-gauge --> dashicons-admin-dashboard
• dashicons-admin-dashboard --> dashicons-admin-home
• dashicons-camera2 --> dashicons-camera
• dashicons-imgedit-undo --> dashicons-undo
• dashicons-imgedit-redo --> dashicons-redo
• dashicons-xit --> dashicons-dismiss
• dashicons-arrow-up --> dashicons-upload
• dashicons-arrow-down --> dashicons-download
• dashicons-network --> dashicons-networking
• dashicons-jobs-developers --> dashicons-hammer
• dashicons-jobs-designers --> dashicons-art
• dashicons-jobs-migration --> dashicons-migrate
• dashicons-jobs-performance --> dashicons-performance
• imgedit --> image, write out full name (rleft --> rotate-left)
• graphs to --> chart-bar, chart-line, chart-area, chart-pie
• change everything with 1/2/3 to alt, alt-2
• change all arr to arrow
• remove twitter-alt b/c it's not correct branding

I also updated the data-uri for the font based on the latest version of dashicons.

[nacin]

In 26752:

Updates to Dashicons classes.

props melchoyce.
fixes #26227.