Make ULS trigger and settings more keyboard accessible

It's now possible to tab to "display settings" and "input settings"
and access them with enter or space. Also escape can now be used
to close the settings screen.

The ULS trigger in interlanguage position is a button to get
native accessibility features.

Also removed `mw.hook( 'mw.uls.settings.open' ).fire( 'uls' );` as
it didn't seem very useful and there wasn't immediately obvious
place to put it. The existing click handler could be removed because
the settings dialog themselves place event listeners.

The patch is a bit longer than strictly necessary because the CSS
was mess (rules in different modules, poorly organized) and I had
to bring related rules together to understand them.

Bug: T52793
Change-Id: Id37c2665b1c97b81ef57be27a1abfae0db6b34d5

resources/css/ext.uls.inputsettings.less

@@ -1,14 +1,5 @@
 @import 'mediawiki.mixins';
 
-.input-settings-block {
-	background: transparent no-repeat left top;
-	.background-image-svg( '../images/input.svg', '../images/input.png' );
-	background-size: 20px auto;
-	color: #222;
-	padding-left: 26px;
-	cursor: pointer;
-}
-
 .imelabel {
 	display: block;
 	padding-bottom: 10px;