Add standard shortcut for "Show/Hide Hidden Files"

Summary:
Add a standard shortcut for this action so it can be synchronized between different apps
and the file dialog; see https://bugs.kde.org/show_bug.cgi?id=262551.

The action in Dolphin also has F8 bound to this, but the definition here only accept two
default shortcuts, and F8 seems kind of random.

CCBUG: 262551

Test Plan: Apply D28803 to KIO and D28804 to Dolphin and test there

Reviewers: dfaure, #frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D28802