Use new "Show/Hide Hidden Files" standard shortcut

Summary:
Note that we lose the F8 shortcut since the KStandardShortcut added in XXXX can only
accommodate toew default shortcuts, and I thought Ctrl+H and Alt+. were the more important
ones. Hopefully that's not too high a price to pay.

Depends on D28802

Test Plan:
- Apply D28802 to kconfig
- Apply this patch to Dolphin, compile, and launch Dolphin
- Hit Ctrl+H or Alt+. and see that hidden files are shown or hidden as approproiate

Reviewers: #dolphin, dfaure, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D28804