Administration: Improve the usage of the button CSS classes.

Introduces some consistency in the usage of the button CSS classes, fixes the
focus style for accessibility and responsiveness of the buttons.

- Adds the `button` class to all primary buttons make them responsive
- Removes all `secondary-button` classes and replaces it with button when needed. `button-secondary` shouldn't be used and exists just for backward compatibility reasons
- Replaces classes inside `submit_button()` with a shorthand for some buttons, and use an empty string for the default `button` class. Passing `button` is unnecessary
- Adjusts `get_submit_button()` to remove empty items

Props iseulde, dimchik, chris_d2d, mhowell, afercia.
Fixes #27314, #37138, #37448.


git-svn-id: https://develop.svn.wordpress.org/trunk@38672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/widgets.php

@@ -254,7 +254,7 @@ function wp_widget_control( $sidebar_args ) {
 		<a class="widget-control-close" href="#close"><?php _e('Close'); ?></a>
 		</div>
 		<div class="alignright<?php if ( 'noform' === $has_form ) echo ' widget-control-noform'; ?>">
-			<?php submit_button( __( 'Save' ), 'button-primary widget-control-save right', 'savewidget', false, array( 'id' => 'widget-' . esc_attr( $id_format ) . '-savewidget' ) ); ?>
+			<?php submit_button( __( 'Save' ), 'primary widget-control-save right', 'savewidget', false, array( 'id' => 'widget-' . esc_attr( $id_format ) . '-savewidget' ) ); ?>
 			<span class="spinner"></span>
 		</div>
 		<br class="clear" />
@@ -278,4 +278,4 @@ function wp_widget_control( $sidebar_args ) {
  */
 function wp_widgets_access_body_class($classes) {
 	return "$classes widgets_access ";
-}
+}