Code review comment for lp://qastaging/~edgar-b-dsouza/acire/save_snippet_as

Changes:
- Implemented Save As button (shortcut Ctrl+S), as per Bug #514124, with gtk.FileChooserDialog; default folder is user home folder, filename is basename of snippet's original filename.
- Implemented copy_snippet() handler for Copy button (though gtksourceview editor already provides copy functionality, the button is single-step vs. Select All+Copy in the editor)
- Added shortcut Ctrl+C for Copy button. Enables snippet copying when editor does not have focus.
- Added shortcut F5 for Execute button. Focus can remain in the editor widget, enables edit-run-fix-rerun with only keyboard.

Please review.

Thanks,
Ed.