Fix disabling of DolphinNewFileMenu

Summary:
Commit e133c455 doesn't work because the actions in the
KNewFileMenu are added only when we show the context menu for the first
time.

Fixes the failing test cases in D16005.

Test Plan: Run `dolphinmainwindowtest` and open the context menu in different places (Home, Root, Trash, `bluetooth:/`, etc.)

Reviewers: #dolphin

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D16006