Add Ctrl+H to the list of shortcuts for "show/hide hidden files"

Summary:
CCBUG: 390527

Like {D10558}, but for the file dialogs too, to maintain consistency.

Test Plan: Tested with Open and Save dialogs in Kate. New and old shortcuts all worked and correctly triggered showing and hiding hidden files

Reviewers: #frameworks, #dolphin, elvisangelaccio, markg

Reviewed By: markg

Subscribers: markg

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D10616