Add actions menu inside content language selector

This patch replaces the display and input settings menu bar at the
bottom of the content language selector, with a floating icon that
opens a menu containing all the available language actions. In case
that only the language settings action is available, the language
settings menu is being opened instead.

In order to provide extensibility and support the addition of new
action items from other extensions, a registry class that inherits
from OO.Registry class is created. This class is used to create a
singleton registry object that holds all action items that should
be rendered inside the menu. Other modules/extensions can use this
registry to add new actions items to the menu, by passing the item
as argument, in the following form:
{ name: "", icon: "", text: "", handler: function() {} }

Bug: T289840
Change-Id: Iee017a9e3e6a654145e9fdd2b7df35baa348697d

resources/css/ext.uls.interface.less

@@ -36,6 +36,23 @@
 	}
 }
 
+.mw-ui-button.uls-language-actions-button {
+	position: absolute;
+	bottom: 0;
+	// set a value that doesn't cause overlap with scrollbar
+	right: 16px;
+	background: center transparent no-repeat;
+	min-width: unset;
+
+	&--single {
+		background-image: url( ../images/cog.svg );
+	}
+
+	&--multiple {
+		background-image: url( ../images/ellipsis.svg );
+	}
+}
+
 .uls-tipsy.uls-tipsy {
 	z-index: 1000;
 }