Properly support opening in browser tabs

[garrett]

When middle-clicking on a folder (or control-clicking), it opens in a new tab, but the directory is not the directory that was clicked on. The new tab is the previous directory, which is unexpected.

[garrett]

Switching to links in the pathbar as mentioned in #641 should fix the issue for the pathbar, at least.

Files will be tricker due to other necessary interactions.

[jelly]

What more is expected here? Note that we do run another bridge in a new window so this will be fairly expensive.

[garrett]

It should have the shell in the new tab after middle-clicking on a link. ("Fairly expensive" is really relative.)