Fix #934: Request to add image bookmarking/tagging within a folder

Fix #934: Request to add image bookmarking/tagging within a folder

https://github.com/BestImageViewer/geeqie/issues/934

The last image viewed in a folder is stored in a list. If the folder is
re-opened, focus moves to the last image viewed.

An option on Preferences/Behavior sets the maximum size of the list.