Cleans up some duplication for each page.

Reviewed-by: Friedemann Kleint

To free up the name for an upcoming VCSBaseOptionsPage class that forms
the base for all version control option pages.

Reviewed-by: Friedemann Kleint

Present user with a checkable list of files to be cleaned (add reusable
dialog to VCSBase module).

Fixes:
- Cannot diff a file that does not belong to a project
- Cannot commit when a temporary diff/log view is open
  due to the current file pointing to a temporary directory
- git's project-related actions not passing the correct
  relative path.

Implementation:
- Centralize code to listen for Qt Creator's relevant state changes
  in VCSBasePlugin, dispatching the changes to the instances affected.
  (avoiding multiple invocations of searches/QFileInfo on current).
- Do the same for the corelistener catching closing SubmitEditors.
- Introduce VCSBasePluginState representing the relevant state
  (current file/project).
- Call git with working directory set and relative arguments
- Remove setEnabled/isEnabled() logic of IVersionControl
- Pass toplevel from VCSManager to avoid duplicate searches.

...and give it some color and formatting.

Task-number: 244831

Hopefully i didn't miss a plugin.

Fixes: Add common configuration to the VCSA base plugin; submit message check script and user name configuration.                                           Details: Extend submit editor widget by configureable fields. Use them in the VCS base submit editor to specify users, provide completion and selection dialog for them.