Also use standard keyboard shortcut for "show/hide hidden files"

Summary:
Use Ctrl+H as another keyboard shortcut to show and hide hidden files, as it is already the standard used in other GTK-based Linux file managers (Nautilus, Nemo, Caja, Thunar, Pantheon Files). This doesn't replace any existing shortcuts, so existing KDE users' muscle memory is retained.

FEATURE: 390527
FIXED-IN: KDE Applications 17.12.3

Test Plan:
- Ctrl+H now shows and hides hidden files
- Alt+. still works to do the same
- F8 still works to do the same
- Toolbar button still changes state appropriately when any of the three shortcuts are used

Reviewers: #dolphin, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: emateli, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D10558