Implement tiling_drag_threshold

Implements `tiling_drag_threshold <threshold>` to prevent accidental
dragging of tiling containers. If a container (and all of its
descendants) are unfocused and the tile bar is pressed, a threshold
will be used before actually starting the drag. Once the threshold has
been exceeded, the cursor will change to the grab icon and the operation
will switch from `OP_MOVE_TILING_THRESHOLD` to `OP_MOVE_TILING`.

sway/commands/tiling_drag_threshold.c

@@ -0,0 +1,22 @@
+#include <string.h>
+#include "sway/commands.h"
+#include "sway/config.h"
+#include "log.h"
+
+struct cmd_results *cmd_tiling_drag_threshold(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "tiling_drag_threshold", EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+
+	char *inv;
+	int value = strtol(argv[0], &inv, 10);
+	if (*inv != '\0' || value < 0) {
+		return cmd_results_new(CMD_INVALID, "tiling_drag_threshold",
+			"Invalid threshold specified");
+	}
+
+	config->tiling_drag_threshold = value;
+
+	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+}