[preview] Add Open Folder menu item , fix bugs

This adds an Open Folder menu item to the right click menu and to the config dialog's default left click actions. Related to gajim#8888 (moved).

The on_save_as_menuitem_activate handler wrongly had _new as suffix, which lead to tracebacks. I removed the prefix.

If Open Link in Browser is selected as default action for left clicking an image, the user would get a warning when clicking an encrypted image. Instead, we fall back to opening the image directly instead of issuing a warning dialog (fixes #323 (closed)).