display: Add new arrangement widget and hook it up

This commits adds a new arrangement widget, refactoring the existing
code and addressing a number of issues:
 * Forced snapping made laying out more than 2 monitors hard
 * Random gaps would be shown between monitors
 * The scaling was fixed and usually tiny

https://bugzilla.gnome.org/show_bug.cgi?id=786971

panels/display/cc-display-arrangement.h

@@ -0,0 +1,40 @@
+/* -*- mode: c; style: linux -*-
+ * 
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * Written by: Benjamin Berg <bberg@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _CC_DISPLAY_ARRANGEMENT_H
+#define _CC_DISPLAY_ARRANGEMENT_H
+
+#include <gtk/gtk.h>
+#include "cc-display-config.h"
+
+G_BEGIN_DECLS
+
+#define CC_TYPE_DISPLAY_ARRANGEMENT cc_display_arrangement_get_type ()
+G_DECLARE_FINAL_TYPE (CcDisplayArrangement, cc_display_arrangement, CC, DISPLAY_ARRANGEMENT, GtkDrawingArea);
+
+CcDisplayArrangement* cc_display_arrangement_new                 (CcDisplayConfig      *config);
+
+CcDisplayMonitor*     cc_display_arrangement_get_selected_output (CcDisplayArrangement *arr);
+void                  cc_display_arrangement_set_selected_output (CcDisplayArrangement *arr,
+                                                                  CcDisplayMonitor     *output);
+
+G_END_DECLS
+
+#endif /* _CC_DISPLAY_ARRANGEMENT_H */