make Webodf version a global property on the new namespace webodf

Commit is part of #465. It is done because with the About dialog of the editor there is another place which would estimate the version. So it is better to have it calculated once and then kept in a central place.

The "webodf" namespace seemed a good spot for it, so is getting introduced by this.
Also is in the long term "webodf" planned to be used as the namespace for all webodf sub-namespaces and variables, to avoid conflicts with other frameworks.

See also discussion in #465