Add print styles to backend

[strarsis]

It would be great if lists and content can be printed from WordPress backend (admin).
Currently the whole UI is printed which wastes space, ink and clutters everything, also content fields are clipped.

A dedicated default print stylesheet for WordPress backend would use a different ink-saving, reading-friendly font, remove all unnecessary UI elements, ensure that all content fields are printed completely.

[sabernhardt]

@strarsis Thanks for the suggestion!

It's worth adding at least a few print styles to WordPress core, to remove the gray background color and hide the navigation items, etc.

I think changing the font would belong better in a plugin, though, and perhaps changing (most) colors would belong there as well.

This patch makes some basic changes for printing. I chose to give the #wpcontent section a rather arbitrary 1em margin on each side so content would not run to the edge of the page.

[isabel_brison]

This is a great improvement! I think we can go a bit further and remove the UI elements from list pages such as posts (search form, bulk actions, pagination buttons, filter fields). Also perhaps it would be worth trying a single-column layout on the homepage, as right now the columns are still overlapping.

Also ran into a strange bug when printing Menus and Settings pages where the writing goes all weird, but it only happens on Chrome/Mac for some reason. It's not related to these changes though.

[isabel_brison]

UI elements can be removed from list pages.

[isabel_brison]

Dashboard columns overlapping

[isabel_brison]

Menus page