Made add and remove operations of ResourceEditor VCS-aware. Also, remove
operation now can remove files from filesystem.

FileUtils::removeFile() and VcsManager::promptToAdd functions were
extracted from ProjectExplorer to prevent code duplication.
RemoveFileDialog was also moved to coreplugin.

Change-Id: Ia51127288030e52ce9475b369e56ea034dfa5d1e
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>