Project: Use Utils::FileName as return type for projectFilePath()

Change-Id: If304d5e95cd9ac714100abaae35d60e3e1050eb2
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Remove some needless ProjectExplorer::

... and use ProjectExplorerPlugin::instance() directly in some places
where a variable was defined for it and used exactly once.

No code change.

Change-Id: I095fc80ac29f717eaabf13afa90c3bf6d9daf90a
Reviewed-by: Daniel Teske <daniel.teske@digia.com>