Implement showing window icons in titlebar (#4439)

This feature defaults to off, and can be turned on for individual windows,
or (with for_window) for all new windows. See the userguide change.

This commit is partially based on work by:

• Marius Muja
• mickael9
• Esteve Varela Colominas
• Bernardo Menicagli

parser-specs/commands.spec

@@ -40,6 +40,7 @@ state INITIAL:
   'scratchpad' -> SCRATCHPAD
   'swap' -> SWAP
   'title_format' -> TITLE_FORMAT
+  'title_window_icon' -> TITLE_WINDOW_ICON
   'mode' -> MODE
   'bar' -> BAR
 
@@ -462,6 +463,20 @@ state TITLE_FORMAT:
   format = string
       -> call cmd_title_format($format)
 
+state TITLE_WINDOW_ICON:
+  'padding'
+    -> TITLE_WINDOW_ICON_PADDING
+  enable = '1', 'yes', 'true', 'on', 'enable', 'active', '0', 'no', 'false', 'off', 'disable', 'inactive'
+    -> call cmd_title_window_icon($enable, 0)
+
+state TITLE_WINDOW_ICON_PADDING:
+  end
+    -> call cmd_title_window_icon($enable, &padding)
+  'px'
+    -> call cmd_title_window_icon($enable, &padding)
+  padding = number
+    ->
+
 # bar (hidden_state hide|show|toggle)|(mode dock|hide|invisible|toggle) [<bar_id>]
 state BAR:
   'hidden_state'