Add support for a custom separator symbol

This introduces a "separator_symbol" property for the i3bar configuration.
If set, the specified string will be used as a separator instead of a vertical line. Since it is an optional configuration, complete backwards compatibility is given.

fixes #1472

parser-specs/config.spec

@@ -370,6 +370,7 @@ state BAR:
   'output'                 -> BAR_OUTPUT
   'tray_output'            -> BAR_TRAY_OUTPUT
   'font'                   -> BAR_FONT
+  'separator_symbol'       -> BAR_SEPARATOR_SYMBOL
   'binding_mode_indicator' -> BAR_BINDING_MODE_INDICATOR
   'workspace_buttons'      -> BAR_WORKSPACE_BUTTONS
   'strip_workspace_numbers' -> BAR_STRIP_WORKSPACE_NUMBERS
@@ -435,6 +436,10 @@ state BAR_FONT:
   font = string
       -> call cfg_bar_font($font); BAR
 
+state BAR_SEPARATOR_SYMBOL:
+  separator = string
+      -> call cfg_bar_separator_symbol($separator); BAR
+
 state BAR_BINDING_MODE_INDICATOR:
   value = word
       -> call cfg_bar_binding_mode_indicator($value); BAR