Administration: Fix minor misalignments caused by the button-link CSS class.

After [40059] the CSS class `button-link` uses `text-align: left` by default.
This change now requires to limit as much as possible the use of `button-link`
to controls that should really look like links and to explicitly set
`text-align: center` in a few other cases.

Fixes #39983.


git-svn-id: https://develop.svn.wordpress.org/trunk@40358 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/customize-nav-menus.css

@@ -150,6 +150,7 @@
 	outline: none;
 	overflow: hidden;
 	cursor: pointer;
+	text-align: center;
 }
 
 .wp-customizer .menu-item.menu-item-edit-active .item-edit .toggle-indicator:after {
@@ -521,6 +522,7 @@
 	box-shadow: none;
 	outline: none;
 	cursor: pointer;
+	text-align: center;
 }
 
 #available-menu-items .accordion-section-title .no-items,
@@ -612,6 +614,7 @@
 	box-shadow: none;
 	outline: none;
 	cursor: pointer;
+	text-align: center;
 }
 
 #available-menu-items .menu-item-handle .item-add:focus {