When the viewer is embedded, change the viewBookmark buttons to open the current view in a new window by default

Currently in the viewer, clicking the viewBookmark "buttons" (they are actually links, styled to look like the other buttons) updates the hash corresponding to the current document position.
By either middle-clicking or right-clicking (and choosing open in new tab/window) the user is able to open the current document position in a new tab/window. This behaviour is probably not very discoverable, but this is most likely a non-issue when the viewer is used "standalone". However, when the viewer is *embedded* in a page, the viewBookmark "buttons" will appear to do nothing when clicked.

This PR changes the behaviour of the viewBookmark "buttons" slightly when the viewer is *embedded*. With this patch, the "buttons" will instead, by default, open the current document position in a new window. To let the user know that this feature exists when the viewer is *embedded*, the strings are also changed slightly to reflect this behaviour.

This PR (together with #3739) should thus fix [bug 921162](https://bugzilla.mozilla.org/show_bug.cgi?id=921162).

/cc @timvandermeij - Since we discussed this issue on IRC.