Add config option workspace_layout

This implements the config parsing of
`workspace_layout <default|stacking|tabbed>`

http://i3wm.org/docs/userguide.html#_layout_mode_for_new_containers

sway/commands.c

@@ -75,6 +75,7 @@ static sway_cmd cmd_splitv;
 static sway_cmd cmd_sticky;
 static sway_cmd cmd_workspace;
 static sway_cmd cmd_ws_auto_back_and_forth;
+static sway_cmd cmd_workspace_layout;
 
 static sway_cmd bar_cmd_binding_mode_indicator;
 static sway_cmd bar_cmd_bindsym;
@@ -1982,6 +1983,24 @@ static struct cmd_results *cmd_workspace(int argc, char **argv) {
 	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
 }
 
+static struct cmd_results *cmd_workspace_layout(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "workspace_layout", EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+
+	if (strcasecmp(argv[0], "default") == 0) {
+		config->default_layout = L_NONE;
+	} else if (strcasecmp(argv[0], "stacking") == 0) {
+		config->default_layout = L_STACKED;
+	} else if (strcasecmp(argv[0], "tabbed") == 0) {
+		config->default_layout = L_TABBED;
+	} else {
+		return cmd_results_new(CMD_INVALID, "workspace_layout", "Expected 'workspace_layout <default|stacking|tabbed>'");
+	}
+	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+}
+
 static struct cmd_results *cmd_ws_auto_back_and_forth(int argc, char **argv) {
 	struct cmd_results *error = NULL;
 	if ((error = checkarg(argc, "workspace_auto_back_and_forth", EXPECTED_EQUAL_TO, 1))) {
@@ -2036,6 +2055,7 @@ static struct cmd_handler handlers[] = {
 	{ "sticky", cmd_sticky },
 	{ "workspace", cmd_workspace },
 	{ "workspace_auto_back_and_forth", cmd_ws_auto_back_and_forth },
+	{ "workspace_layout", cmd_workspace_layout },
 };
 
 static struct cmd_results *bar_cmd_binding_mode_indicator(int argc, char **argv) {

sway/sway.5.txt

@@ -208,6 +208,9 @@ or triggered at runtime.
 	Specifies that the workspace named _name_ should appear on the specified
 	_output_.
 
+**workspace_layout** <default|stacking|tabbed>::
+	Specifies the start layout for new workspaces.
+
 Criteria
 --------