Add new bar.binding_mode_indicator configuration.

i3 current behavior hides the binding mode indicator when
workspace buttons are disabled.
This patch adds a new configuration for i3bar called
'binding_mode_indicator' which acts like the workspace_buttons.
It is now possible to configure i3bar to hide the
workspace buttons and keep showing the binding mode indicator.
This should make the hide workspace buttons configuration
more convenient for those who are heavily using binding
modes.
Default value for binding_mode_indicator is true.

docs/ipc

@@ -494,6 +494,8 @@ font (string)::
 	The font to use for text on the bar.
 workspace_buttons (boolean)::
 	Display workspace buttons or not? Defaults to true.
+binding_mode_indicator (boolean)::
+	Display the mode indicator or not? Defaults to true.
 verbose (boolean)::
 	Should the bar enable verbose output for debugging? Defaults to false.
 colors (map)::
@@ -539,6 +541,7 @@ urgent_workspace_text/urgent_workspace_bar::
  "status_command": "i3status",
  "font": "-misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1",
  "workspace_buttons": true,
+ "binding_mode_indicator": true,
  "verbose": false,
  "colors": {
    "background": "#c0c0c0",

docs/userguide

@@ -1180,11 +1180,32 @@ workspace_buttons <yes|no>
 --------------------------
 
 *Example*:
---------------------
+------------------------
 bar {
     workspace_buttons no
 }
---------------------
+------------------------
+
+=== Binding Mode indicator
+
+Specifies whether the current binding mode indicator should be shown or not.
+This is useful if you want to hide the workspace buttons but still be able
+to see the current binding mode indicator.
+For an example of a +mode+ definition, see <<resizingconfig>>.
+
+The default is to show the mode indicator.
+
+*Syntax*:
+-------------------------------
+binding_mode_indicator <yes|no>
+-------------------------------
+
+*Example*:
+-----------------------------
+bar {
+    binding_mode_indicator no
+}
+-----------------------------
 
 === Colors