Appwizard: store chosen vcsPlugin in developer .kdev4 file

Summary:
Currently the information about the vcsPlugin is dropped
after the creation step is done, and the plugin to use only guessed
by KDevelop::ProjectPrivate::loadVersionControlPlugin(...)

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5513