Add non-modal stash management dialog and additional menu option
"Stash snapshot..." to stash away changes prompting for a description,
which will immediately replay the stash (take snapshot and continue
working).
Add interface to IVersionControl for creating/restoring/deleting
snapshots for backup/complex undo operations (currently supported
by git only). Add test options to VCSBasePlugin.
Clean up and extend git client accordingly.

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.

  * Use id() for methods returning a string used to represent
    some type of object.
  * Use displayName() for strings that are meant to be user
    visible.
  * Quieten some warnings while touching the files anyway.
  * Move Factories to their products in the plugins where that
    was not done before.

Reviewed-by: dt

Make use of VCS base class.

Add a QVariant cookie to HgTask to be emitted on successful
execution, wired to IVersionControl.