Add new bar.binding_mode_indicator configuration.

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.