Improve Ux for the places panel's hidden items

Summary:
BUG: 400860
FIXED-IN: 18.12.0

Clean up the context menu for the places panel.

• Change Text "Show All Entries" -> "Show Hidden Places"
• Use State dependent icon (like hidden files)
• Disable instead of hide if not places are hidden.
• Toggle to unchecked if last item of group is unhidden.

Create a copy of this "Show Hidden Places" entry in the main dolphin menu View->Places.

Test Plan:
With Hidden Places

Context Menu:

Without Hidden Places.

Reviewers: Dolphin, VDG, elvisangelaccio, ngraham

Reviewed By: Dolphin, VDG, elvisangelaccio, ngraham

Subscribers: cfeck, elvisangelaccio, veqz, davidc, ngraham, kfm-devel

Tags: Dolphin

Maniphest Tasks: T8349

Differential Revision: https://phabricator.kde.org/D16767