Alter config variable replacement process

Currently, variables cannot contain commands and cannot span more than
one argument. This is due to variable replacement happening after
determining the handler and after splitting the config line into
arguments.

This changes the process to:
0. Check for empty lines and block boundaries
1. Split the arguments as before
2. Verify that the first argument is not a variable. If needed the
following occurs
    a. Perform variable replacement on just the first argument
    b. Join the arguments back together then split the arguments again. This is needed when the variable
contains the command and arguments for the command.
3. Determine the handler
4. If the handler is cmd_set, escape the variable name so that it does
not get replaced
5. Join the arguments back together, do variable replacement on the full
command, and split the arguments again
6. Perform any needed quote stripping or unescaping on arguments
7. Run the command handler

This allows for config snippets such as:

```
set $super bindsym Mod4
$super+a exec some-command
```

and

```
set $bg bg #ffffff solid_color
output * $bg
```

sway/commands.c

@@ -353,12 +353,14 @@ struct cmd_results *config_command(char *exec) {
 	struct cmd_results *results = NULL;
 	int argc;
 	char **argv = split_args(exec, &argc);
+
+	// Check for empty lines
 	if (!argc) {
 		results = cmd_results_new(CMD_SUCCESS, NULL, NULL);
 		goto cleanup;
 	}
 
-	// Start block
+	// Check for the start of a block
 	if (argc > 1 && strcmp(argv[argc - 1], "{") == 0) {
 		char *block = join_args(argv, argc - 1);
 		results = cmd_results_new(CMD_BLOCK, block, NULL);
@@ -366,22 +368,54 @@ struct cmd_results *config_command(char *exec) {
 		goto cleanup;
 	}
 
-	// Endblock
+	// Check for the end of a block
 	if (strcmp(argv[argc - 1], "}") == 0) {
 		results = cmd_results_new(CMD_BLOCK_END, NULL, NULL);
 		goto cleanup;
 	}
-	wlr_log(WLR_INFO, "handling config command '%s'", exec);
+
+	// Make sure the command is not stored in a variable
+	if (*argv[0] == '$') {
+		argv[0] = do_var_replacement(argv[0]);
+		char *temp = join_args(argv, argc);
+		free_argv(argc, argv);
+		argv = split_args(temp, &argc);
+		free(temp);
+		if (!argc) {
+			results = cmd_results_new(CMD_SUCCESS, NULL, NULL);
+			goto cleanup;
+		}
+	}
+
+	// Determine the command handler
+	wlr_log(WLR_INFO, "Config command: %s", exec);
 	struct cmd_handler *handler = find_handler(argv[0], NULL, 0);
-	if (!handler) {
+	if (!handler || !handler->handle) {
 		char *input = argv[0] ? argv[0] : "(empty)";
-		results = cmd_results_new(CMD_INVALID, input, "Unknown/invalid command");
+		char *error = handler
+			? "This command is shimmed, but unimplemented"
+			: "Unknown/invalid command";
+		results = cmd_results_new(CMD_INVALID, input, error);
 		goto cleanup;
 	}
-	int i;
-	// Var replacement, for all but first argument of set
-	// TODO commands
-	for (i = handler->handle == cmd_set ? 2 : 1; i < argc; ++i) {
+
+	// Do variable replacement
+	if (handler->handle == cmd_set && argc > 1 && *argv[1] == '$') {
+		// Escape the variable name so it does not get replaced by one shorter
+		char *temp = calloc(1, strlen(argv[1]) + 2);
+		temp[0] = '$';
+		strcpy(&temp[1], argv[1]);
+		free(argv[1]);
+		argv[1] = temp;
+	}
+	char *command = do_var_replacement(join_args(argv, argc));
+	wlr_log(WLR_INFO, "After replacement: %s", command);
+	free_argv(argc, argv);
+	argv = split_args(command, &argc);
+	free(command);
+
+	// Strip quotes and unescape the string
+	for (int i = handler->handle == cmd_set ? 2 : 1; i < argc; ++i) {
 		if (handler->handle != cmd_exec && handler->handle != cmd_exec_always
 				&& handler->handle != cmd_bindsym
 				&& handler->handle != cmd_bindcode
@@ -389,14 +423,11 @@ struct cmd_results *config_command(char *exec) {
 				&& (*argv[i] == '\"' || *argv[i] == '\'')) {
 			strip_quotes(argv[i]);
 		}
-		argv[i] = do_var_replacement(argv[i]);
 		unescape_string(argv[i]);
 	}
-	if (handler->handle) {
+
+	// Run command
 	results = handler->handle(argc - 1, argv + 1);
-	} else {
-		results = cmd_results_new(CMD_INVALID, argv[0], "This command is shimmed, but unimplemented");
-	}
 
 cleanup:
 	free_argv(argc, argv);