Enable all file actions on helm-projectile sources

Revamp the source definition for helm-projectile file sources to enable
all regular file operations on them:

* Remove the (action . (lambda () ...)) form from the projectile-files
  and projectile-recentf sources: this shadowed all other file actions.
* Introduce a new variable, helm-projectile-current-project-root, make
  it buffer-local to the candidate buffer and set it to the project root
  for each helm-projectile session.
* Rename the helm-projectile candidate buffer such that there is a
  unique buffer per project dir.
* Set the help-message and mode-line properties of file sources
  to mirror those of helm-ff and helm-files.
* Introduce the coerce form to file source definitions to make the
  candidate file names into absolute file names (required for e.g. the
  Dired action).