placesview: add view for fixed drives and networks

Places sidebar shows XDG directories, mounted and unmounted devices,
connected networks, bookmarks and actions like 'Connect to server'
and 'Insert location', which causes the sidebar to grow very quickly
and look cluttered. Because of that, new mockups for the sidebar try
to simplify it.

To make the sidebar simpler, the new mockups propose that it should
only handle connected networks and removable devices such as flash
drives and USB devices, and delegates other devices for external
widgets through the 'Other Locations' item.

To handle fixed devices and manage network connections, add a new
widget named GtkPlacesView, based on Nautilus mockups to keep
consistency between GNOME file management tools - in this case,
between Nautilus and the bundled Gtk's file chooser.

https://bugzilla.gnome.org/show_bug.cgi?id=752034

tests/testplacesview.c

@@ -0,0 +1,24 @@
+#include <gtk/gtk.h>
+
+int
+main (int argc, char *argv[])
+{
+  GtkWidget *win;
+  GtkWidget *view;
+
+  gtk_init (&argc, &argv);
+
+  win = gtk_window_new (GTK_WINDOW_TOPLEVEL);
+  gtk_window_set_default_size (GTK_WINDOW (win), 400, 600);
+
+  view = gtk_places_view_new ();
+
+  gtk_container_add (GTK_CONTAINER (win), view);
+  gtk_widget_show_all (win);
+
+  g_signal_connect (win, "delete-event", G_CALLBACK (gtk_main_quit), win);
+
+  gtk_main ();
+
+  return 0;
+}