Prevent the Extra Arguments ComboBox to Stretch Too Much

Summary: This basically ports d56117d6 to the build dir chooser.

Test Plan: Create a new project from template (pick any template, any valid name) and add some ridiculously long cmake extra arguments (say, -DTEST=<really long string>), then create a new project again and look at the size of the Extra Arguments combo box.

Reviewers: #kdevelop, mwolff

Reviewed By: #kdevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D18414