Update Finnish translation for 5.4

Also fix minor mistake in string "Files of type:".

Consistency checks (source -> translation):
* socket -> pistoke
* escape sequence -> ohjauskoodi
* (XML) tag -> tunniste
* XML declaration -> XML-esittely

Change-Id: Ia530f8bb0badea488afc7fb45dc62100b4f1dd96
Reviewed-by: Aki Koskinen <qt@akikoskinen.info>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>