Abstract binding configuration to bindings.[ch]

Create files bindings.[ch] to contain functions for configuring,
finding, and running bindings.

Use the new function `configure_binding` for binding configuration. This
function adds a binding from config parameters.

Export the function `modifiers_from_str` from config_directives.h.

This change is made in preparation for the new bindmouse functionality.

include/bindings.h

@@ -0,0 +1,25 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009-2014 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * bindings.h: Functions for configuring, finding, and running bindings.
+ *
+ */
+#pragma once
+
+/**
+ * The name of the default mode.
+ *
+ */
+const char *DEFAULT_BINDING_MODE;
+
+/**
+ * Adds a binding from config parameters given as strings and returns a
+ * pointer to the binding structure. Returns NULL if the input code could not
+ * be parsed.
+ *
+ */
+Binding *configure_binding(const char *bindtype, const char *modifiers, const char *input_code,
+        const char *release, const char *command, const char *mode);