Implement title alignment

This adds support for `i3 4.16`'s ability to set the title alignment.
The command is `title_align left|center|right`.

When the title is on the right, marks are moved to the left. Otherwise,
they are on the right.

include/sway/commands.h

@@ -172,6 +172,7 @@ sway_cmd cmd_swaybg_command;
 sway_cmd cmd_swaynag_command;
 sway_cmd cmd_swap;
 sway_cmd cmd_tiling_drag;
+sway_cmd cmd_title_align;
 sway_cmd cmd_title_format;
 sway_cmd cmd_titlebar_border_thickness;
 sway_cmd cmd_titlebar_padding;

include/sway/config.h

@@ -357,6 +357,12 @@ enum mouse_warping_mode {
 	WARP_CONTAINER
 };
 
+enum alignment {
+	ALIGN_LEFT,
+	ALIGN_CENTER,
+	ALIGN_RIGHT
+};
+
 /**
  * The configuration struct. The result of loading a config file.
  */
@@ -409,6 +415,7 @@ struct sway_config {
 	bool validating;
 	bool auto_back_and_forth;
 	bool show_marks;
+	enum alignment title_align;
 	bool tiling_drag;
 
 	bool smart_gaps;