Rather than using the search entry text as a single term when filtering
panels, split the text into multiple terms and require result to match all
of them, which is consistent with the search provider and the encouraged
search pattern for GNOME applications.
https://bugzilla.gnome.org/show_bug.cgi?id=694960
Keep history of the visited panels, when navigating between
them through links, or activated through the shell's menu items,
and go back to the overview when the history is empty.
https://bugzilla.gnome.org/show_bug.cgi?id=643322
So that we can more easily add GdHeaderBar in the following
patches.
Much of the advantage of using gtkbuilder was lost anyway because
we couldn't really use it from glade without corrupting the file.
https://bugzilla.gnome.org/show_bug.cgi?id=692174
