Add All Top-Level Targets to the Menu

Summary:
The previous code did not handle folders with
multiple actions but no parent.

Test Plan:
Create a project with two toplevel targets (e.g.
a CMakeLists.txt with two add_executable calls), add the
project to a KDevelo session, go to Configure Launches and
try adding a compiled binary by clicking on the Add button
at the top-left corner. Before this patch, only one of the
two toplevel targets would show in the menu.

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: kfunk, apol, arrowd, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D18021