File templates tool: with no open document, use base dir of project or home

Summary:
Currently without any document open, the base dir used for the newly
created files is an invalid empty url and in debug builds also triggers
an assert in TemplateClassGenerator constructor.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, flherne, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4192