Consolidate buttons CSS into a single file

[koopersmith]

The button updates added in #21598 have made the code quite modular, but it is currently split between wp-admin.css and the two colors files (which have identical copies of the bulk of the buttons CSS). This is not particularly maintainable, and makes it impossible to use the buttons outside the context of the admin.

Given our plans for #21813, we'll need to reuse this CSS inside TinyMCE, and redeclaring the same code for the third time just seems silly.

[koopersmith]

In [22236]:

Consolidate buttons CSS into a single file.

Adds wp-includes/css/buttons.css to consolidate buttons files and make them available in a modular fashion. Buttons are included automatically with the colors styles, and can be included independently by running:

wp_enqueue_style('buttons');

fixes #22197, see #21598.

[koopersmith]

In [22239]:

Revise and combine button selectors.

• Simplifies button selectors. Maintains backwards compatibility for several classes and makes creating new button styles considerably easier.
• Adds buttons documentation.
• Raw input submit/reset/button types now use default browser styles (instead of half-styled borders that did not look like buttons).
• Moves basic button color standardization to the beginning of wp-admin.

see #21598, #22197.

[ocean90]

Can we please remove the -ms- prefixed gradients?