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/css/customize-controls.css

@@ -1204,7 +1204,7 @@ body.cheatin p {
  * Widgets and Menus common styles
  */
 
-/* higher specificity than .wp-core-ui .button-secondary */
+/* higher specificity than .wp-core-ui .button */
 #customize-theme-controls .add-new-widget,
 #customize-theme-controls .add-new-menu-item {
 	cursor: pointer;

src/wp-admin/css/ie.css

@@ -221,8 +221,7 @@ table.fixed td {
 }
 
 #wpbody-content input.button,
-#wpbody-content input.button-primary,
-#wpbody-content input.button-secondary {
+#wpbody-content input.button-primary {
 	overflow: visible;
 }
 
@@ -447,8 +446,8 @@ div#dashboard-widgets {
 	display: block;
 }
 
-.tablenav .button-secondary,
-.nav .button-secondary {
+.tablenav .button,
+.nav .button {
 	padding-top: 2px;
 	padding-bottom: 2px;
 }
@@ -507,7 +506,7 @@ table.ie-fixed {
 	border-collapse: collapse;
 }
 
-.tablenav a.button-secondary {
+.tablenav a.button {
 	display: inline-block;
 	padding: 2px 5px;
 }

src/wp-admin/css/install.css

@@ -443,7 +443,7 @@ body.language-chooser {
 	margin-right: 15px;
 }
 
-.button-secondary.hide-if-no-js,
+.button.hide-if-no-js,
 .hide-if-no-js {
 	display: none;
 }

src/wp-admin/css/list-tables.css

@@ -656,6 +656,7 @@ th.asc a:focus span.sorting-indicator {
 	margin-left: 2px;
 }
 
+.tablenav a.button,
 .tablenav a.button-secondary {
 	display: block;
 	margin: 3px 8px 0 0;

src/wp-admin/css/themes.css

@@ -114,7 +114,7 @@
 	margin-right: 3px;
 }
 
-.theme-browser .theme .theme-actions .button-secondary {
+.theme-browser .theme .theme-actions .button {
 	float: none;
 	margin-left: 3px;
 }