Fix random order in extension menus

Summary:
This patch fixes random order of extension menu items between different runs of KDevelop.

Old behavior leads for example to random reordering of "Analyze With" submenu:

1. "Cppcheck" actions, "Vera++" actions - first run
2. "Vera++" actions, "Cppcheck" actions - some other run.

Test Plan: Tested with master branch

Reviewers: kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3076