Implement fake-outputs option (cmdline, cfg) for multi-monitor testing

This kills the dependency on xdmx and makes the testsuite simpler
and more flexible (in the output sizes / configurations).

include/fake_outputs.h

@@ -0,0 +1,23 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009-2012 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * Faking outputs is useful in pathological situations (like network X servers
+ * which don’t support multi-monitor in a useful way) and for our testsuite.
+ *
+ */
+#ifndef _FAKE_OUTPUTS_H
+#define _FAKE_OUTPUTS_H
+
+/**
+ * Creates outputs according to the given specification.
+ * The specification must be in the format wxh+x+y, for example 1024x768+0+0,
+ * with multiple outputs separated by commas:
+ *   1900x1200+0+0,1280x1024+1900+0
+ *
+ */
+void fake_outputs_init(const char *output_spec);
+
+#endif