VCS: Introduce Base class for VCS plugins, use in git.
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.