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

testcases/t/116-nestedcons.t

@@ -73,6 +73,7 @@ my $expected = {
     workspace_layout => 'default',
     current_border_width => -1,
     marks => $ignore,
+    window_icon_padding => -1,
 };
 
 # a shallow copy is sufficient, since we only ignore values at the root