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

docs/userguide

@@ -585,6 +585,16 @@ workspace_layout default|stacking|tabbed
 workspace_layout tabbed
 ---------------------
 
+=== Window title alignment
+
+This option determines the window title's text alignment.
+Default is +left+
+
+*Syntax*:
+---------------------------------------------
+title_align left|center|right
+---------------------------------------------
+
 === Default border style for new windows
 
 This option determines which border style new windows will have. The default is