closing tabs causes unexpected behavior

[Aeva]

This tracks a loose end from #7 and #14.

This bug effects universe that have at least two browser tabs in them.

In the first situation, if you attempt to destroy the universe from the universe context menu, the browser will segfault and close. fix't

In the second situation, if you close one of the tabs, the other tabs in that universe will no longer display correctly. Tabs in other universes seem to be unaffected. I think one of several things is going on:

• the teardown event on the BrowserWorker inadvertently closes all webkit instances, and...????
• the plug/socket is not being shut down correctly, and that affects all of the other plug/sockets associated to the same process?
• the universe subprocess was terminated somehow

[Aeva]

For the segfault, this is sometimes printed: Gtk:ERROR:gtktreestore.c:592:gtk_tree_store_get_path: assertion failed: (G_NODE (iter->user_data)->parent != NULL)

The last successful call before the segfault appears to be the self.tab_store.remove(tab_iter) in BrowserWindow::close_tab!

So, the solution looks to be to not cache the tab_iter value, but regenerate it as needed.

[Aeva]

Ok, that resolves the segfault, but the other issue remains.