Add '--release' flag for bindsym in the bar block

i3bar's handle_button is modified to also handle XCB_BUTTON_RELEASE
events. During these button release events, only custom commands are
checked to avoid sending multiple workspace ipc messages.

The way this patch is implemented will allow to assign a custom command
for both the press and release of the same button:
bar {
  ...
  bindsym buttonX exec command1
  bindsym --release buttonX exec command2
}

Fixes #3068.

parser-specs/config.spec

@@ -501,12 +501,16 @@ state BAR_WHEEL_DOWN_CMD:
       -> call cfg_bar_wheel_down_cmd($command); BAR
 
 state BAR_BINDSYM:
+  release = '--release'
+      ->
   button = word
       -> BAR_BINDSYM_COMMAND
 
 state BAR_BINDSYM_COMMAND:
+  release = '--release'
+      ->
   command = string
-      -> call cfg_bar_bindsym($button, $command); BAR
+      -> call cfg_bar_bindsym($button, $release, $command); BAR
 
 state BAR_POSITION:
   position = 'top', 'bottom'