Update window title and tab label on document load and tab select; b=356959, r=brade (thanks tortoisecvs for hiding the added file...)

composer/base/locale/en-US/composer/composer.properties

@@ -0,0 +1,5 @@
+untitled=(Untitled)
+titleModifier=Composer
+# in the following string, %t represents the title of the page or its URL
+# and %b the titleModifier above
+titleFormat=%S - %S