EXT-8357 FIXED ESC key sometimes failing to de-focus the sidebar.

I've found out that the only case when pressing ESC doesn't transfer focus from side tray to game area is
when a flat list view (people, groups, teleport history, etc) is focused.
So, I fixed LLFlatListView to handle pressing ESC as expected.

Reviewed by Sergey Litovchuk at https://codereview.productengine.com/secondlife/r/862/

--HG--
branch : product-engine

indra/llui/llflatlistview.cpp

@@ -669,6 +669,14 @@ BOOL LLFlatListView::handleKeyHere(KEY key, MASK mask)
 			}
 			break;
 		}
+		case KEY_ESCAPE:
+		{
+			if (mask == MASK_NONE)
+			{
+				setFocus(FALSE); // pass focus to the game area (EXT-8357)
+			}
+			break;
+		}
 		default:
 			break;
 	}