Add title_align config directive

Controls the window titles alignment in title bars. Possible values are:
- left
- center
- right

Co-authored-by: Orestis Floros <orestisf1993@gmail.com>
- Made title_align a config directive instead of a command. Helps with
some tree_render() issues we had.
- Made title_max_width the same for all 3 cases.
- Modified title offset calculations and added explanations for each
case.
- Append title_padding to mark_width if a mark exists.

Fixes #1750

include/config_directives.h

@@ -56,6 +56,7 @@ CFGFUN(disable_randr15, const char *value);
 CFGFUN(fake_outputs, const char *outputs);
 CFGFUN(force_display_urgency_hint, const long duration_ms);
 CFGFUN(focus_on_window_activation, const char *mode);
+CFGFUN(title_align, const char *alignment);
 CFGFUN(show_marks, const char *value);
 CFGFUN(hide_edge_borders, const char *borders);
 CFGFUN(assign_output, const char *output);