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

Summary:
CCBUG: 390527

Like D10558: Also use standard keyboard shortcut for "show/hide hidden files", 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