Set description in Picasa export only if it is defined

Otherwise Picasa displays filenames as photo descriptions, which is not nice in exported albums
Use Google's terminology in variable names - title/summary, instead of caption/description

Title is the filename and summary is something that is displayed along with the photo on the web (xmp title).
Fix #9665: Allow toggling filmstrip in map view

Maybe we should move that code to lib/tools/filmstrip.c?
Merge pull request #315 from angryziber/master

Improve Picasa export: do not set irrelevant summary for photos consisting of a filename, keep it empty if xmp title is not defined instead
allow longer string length for camera maker/model and lens name

affects cached image data and lens correction module. should fix #9644.
focus: clip focus regions to cropped image.
focus: slight performance fix.
focus: don't display stale old clusters
Fix some strings marked for translation

YEAH! Finally I found time to fix some bogus strings before string
freeze. :)
[l10n]: Update of German translation