Summary:
This is consistent with the HIG, and the 'Rename' entry in
the context menu already behaves like that.

Test Plan:
Right click on /home. The context menu should contained
the 'Delete' entry, but it should be disabled.

Reviewers: #dolphin, #vdg, ngraham

Reviewed By: #dolphin, #vdg, ngraham

Subscribers: ngraham, romangg, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D17012