7 preferences

* Development of the preferences widget
* Close #7

See merge request !5

Protect the field default values against callable.

Close #3

See merge request !3

Once the production release is tagged, push the tag to the remote
repository.

1 branching model

* Reopen the branch 1-branching-model and the corresponding issue.
* The issue #1 has been closed to soon since the script ``build_version.py`` has to be adapted to the imporved branching model.
* This is performad by this merge request
* Closes #1.

See merge request !2

Adapt the branchs model to gitlab (master, production, ...).

the reference manual located in plugin_dbui_book has been moved to the
plugin_dbui documentation.