Add standard shortcut for "Show/Hide Hidden Files"
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.
Reviewers: dfaure, Frameworks
Reviewed By: dfaure
Differential Revision: https://phabricator.kde.org/D28802