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.

include/configuration.h

@@ -384,6 +384,9 @@ struct Barbinding {
     /** The command which is to be executed for this button. */
     char *command;
 
+    /** If true, the command will be executed after the button is released. */
+    bool release;
+
     TAILQ_ENTRY(Barbinding)
     bindings;
 };