make stage handling more consistent

previously the following erroneous scenario was possible:

-  Add a file to stage via :ApexStageAdd
- open stage via :ApexStage
- Switch to another buffer ( do not close stage buffer with :bd )
- Try to add another file to stage via :ApexStageAdd
- open stage via :ApexStage
--- Expected - both files are in stage buffer
--- Actual: only the original file is visible in stage buffer