Improve Picasa export: do not set irrelevant summary for photos consisting of a filename, keep it empty if xmp title is not defined instead

Current master sets Picasa title and summary to the same value (filename without extension in case xmp title is not defined). This doesn't look nice, especially in case of an album where only some photos have nice titles (other will then contain ugly "technical" filenames).

According to Google's documentation, title must be set to the filename in any case and summary is a human-readable caption of the photo. This pull request does exactly that.

In addition (with the second commit), I have renamed variables and parameters to title/summary so that they are not mixed up again.