Add infrastructure for a GNOME Shell search provider

Add a stubbed-out GNOME Shell search provider that currently just matches
for foo.example.com and does nothing when activated.

The XML for the search provider interface is imported to avoid creating
a build-dependency on GNOME Shell.

https://bugzilla.gnome.org/show_bug.cgi?id=712345
Hook GNOME Shell search provider up to tab hostnames

Search windows and tabs for a tab with a hostname that matches the
search string, and switch to that tab and window when the search
result is activated.

Currently, when multiple tabs match, only one search result is shown
and a random tab is selected; it's probably best to switch to the
last-active tab for that hostname rather than a random one.

https://bugzilla.gnome.org/show_bug.cgi?id=712345
Display connected hostname in tab label