Put each module in its own subdirectory

modules/i3/i3.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "../../module.h"
+#include "../../particle.h"
+
+/* Maps workspace name to a content particle. */
+struct i3_workspaces {
+    const char *name;
+    struct particle *content;
+};
+
+struct module *module_i3(
+    struct i3_workspaces workspaces[], size_t workspace_count,
+    int left_spacing, int right_spacing);