Use javascript 1.7 to use 'yield' for PlacesBackups.getBookmarksTree()

PlacesUtils.serializeNodeAsJSONToOutputStream() was deprecated.
It can be replaced by PlacesBackups.getBookmarksTree(), but it requires
the asynchronous processing.

This introduces javascript 1.7 so that bkviewer.js can process the result of
getBookmarksTree() like BookmarkJSONUtils.exportToFile().
Moves the processings at the final phase to the callback 'fini'

This adds a new callback to show_bookmarks. It is called after making
all categories.

This is one of preparation to make show_bookmarks() asynchronous.
Replaces serializeNodeAsJSONToOutputStream() by getBookmarksTree()

PlacesUtils.serializeNodeAsJSONToOutputStream() was deprecated from Firefox30.
Once it seemed to be moved to BookmarkJSONUtils, but finally removed.

This replaces it by PlacesBackups.getBookmarksTree().
It is asynchronous function, then this makes show_bookmarks() asynchronous
processing.