Fix disabling of DolphinNewFileMenu

Summary:
Commit e133c4557ecc37ed3f7e1b9 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