pull: gather all bookmark-pulling code together Pulling bookmarks is done in two rounds. First we do a simple update, then we use the content of the `bookmark` argument to possibly overwrite some bookmark with their remote location. The second step was not done right after the first one for some obscure reason. We now perform them one after the other.

push: perform bookmark export in the push function This part is responsible for adding new bookmarks on the remote. Before that, it was done on its own in `commands.push`. The export is still not integrated with the rest of the push process, but at least it now dwells in the right function.

push: add `pushoperation.bkresult` This attribute will be used to carry the result of the bookmark push (when the `push` function will effectively be doing any bookmarks pushes)

push: pass list of bookmark to `exchange.push` Currently stored but not unused. This parameter will control bookmarks explicitly pushed (added to the server if missing).