introduced i3 command for changing the hidden state and the mode of i3bar

The hidden_state and mode of each i3bar instance can now be controlled from within i3.
Therefore, two new i3 command were introduced:
    _
    bar hidden_state show|hide|toggle [<bar_id>]

    show: always show the bar
    hide: normal hide mode
    toggle: toggle between show and hide (individually for each bar)
    _
    bar mode dock|hide|invisible|toggle [<bar_id>]

    hide,dock: like before
    invisible: always keep the bar hidden
    toggle: toggle between dock and hide (individually for each bar)

This patch introduces a hidden_state ("hidden_state hide|show") in the
barconfig, which indicates the current hidden_state of each i3bar
instance. It only affects the bar when in hide mode. Additionally, a new
invisible mode was introduced. In order to change the hidden_state or
mode of the bar from i3, a barconfig-update event was introduced, for
which a bar can subscribe and the bar then gets notified about the
currently set hidden_state and mode in its barconfig.

For convenience, an id field ("id <bar_id>") was added to the barconfig, where one can
set the desired id for the corresponding bar. If the id is not specified, i3 will
deterministically choose an id; otherwise, with the previous random approach for finding
a new id, which is actually not shared with i3bar, as it would determine its id on
startup, the event-subscription would be destroyed on reload. Still, this issue remains
when manually changing the bar_id in the config and then reloading.

fixes #833, #651

parser-specs/config.spec

@@ -349,6 +349,8 @@ state BAR:
   'status_command'    -> BAR_STATUS_COMMAND
   'socket_path'       -> BAR_SOCKET_PATH
   'mode'              -> BAR_MODE
+  'hidden_state'      -> BAR_HIDDEN_STATE
+  'id'                -> BAR_ID
   'modifier'          -> BAR_MODIFIER
   'position'          -> BAR_POSITION
   'output'            -> BAR_OUTPUT
@@ -378,9 +380,17 @@ state BAR_SOCKET_PATH:
       -> call cfg_bar_socket_path($path); BAR
 
 state BAR_MODE:
-  mode = 'dock', 'hide'
+  mode = 'dock', 'hide', 'invisible'
       -> call cfg_bar_mode($mode); BAR
 
+state BAR_HIDDEN_STATE:
+  hidden_state = 'hide', 'show'
+      -> call cfg_bar_hidden_state($hidden_state); BAR
+
+state BAR_ID:
+  bar_id = word
+      -> call cfg_bar_id($bar_id); BAR
+
 state BAR_MODIFIER:
   modifier = 'Mod1', 'Mod2', 'Mod3', 'Mod4', 'Mod5', 'Control', 'Ctrl', 'Shift'
       -> call cfg_bar_modifier($modifier); BAR