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
{F6398756}
{F6398759}
Context Menu:
{F6398762}
{F6398761}

Without Hidden Places.
{F6414694}
{F6414696}

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