Added a basic routine the check if there is some new version on
pgModeler site. The method makes a request to a specific url passing the
current app version and the must recent version is returned compared to
the passed one.
Changed the update notifier from a standard message box to a more
elaborated floating widget.

Added an option to disable update checking at startup.
Update on CHANGELOG.md and README.md for release 0.7.2-alpha
Merge branch 'update-check-feature' into develop