Battery: Split operating system specific code

This removes all operating system specific code from the main
battery file into their own source files. CMake will add the
correct implementation automatically.

src/battery/dummy.c

@@ -0,0 +1,40 @@
+/**************************************************************************
+*
+* Tint2 : Dummy battery (non-functional)
+*
+* Copyright (C) 2015 Sebastian Reichel <sre@ring0.de>
+*
+* This program is free software; you can redistribute it and/or
+* modify it under the terms of the GNU General Public License version 2
+* or any later version as published by the Free Software Foundation.
+*
+* 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, write to the Free Software
+* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+**************************************************************************/
+
+#include <string.h>
+#include "common.h"
+#include "battery.h"
+
+#warning tint2 has no battery support for this operating system!
+
+gboolean battery_os_init() {
+	return FALSE;
+}
+
+void battery_os_free() {
+	return;
+}
+
+int battery_os_update(struct batstate *state) {
+	return -1;
+}
+
+char* battery_os_tooltip() {
+	return strdup("Operating System not supported");
+}