Add repository creation operation to IVersionControl, implement for hg
and git, add convenience slot with prompts to VCSBasePlugin.
Add respective menu options and make menus are visible in case no VCS is
active.
Change project wizards extension page to list VCS that are capable of
repository creation in a QComboBox in case the directory is not managed
by another VCS (in which case it lists that one for operation 'add').
On that occasion, polish the Project selection to use a QComboBox as
well and add some smartness to find the most suitable project to add via
path matching.

...and its dialog prompt consistently. Rename method and
introduce overload taking IVersionControl. Add convenience slot to
VCSBasePlugin.

Reviewed-by: Friedemann Kleint

Polish: Canconical slots, const-correctness, some QLatin1Strings/tr,