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.

docs/userguide

@@ -1377,7 +1377,7 @@ and will be removed in a future release. We strongly recommend using the more ge
 
 *Syntax*:
 ----------------------------
-bindsym button<n> <command>
+bindsym [--release] button<n> <command>
 ----------------------------
 
 *Example*:
@@ -1385,6 +1385,8 @@ bindsym button<n> <command>
 bar {
     # disable clicking on workspace buttons
     bindsym button1 nop
+    # Take a screenshot by right clicking on the bar
+    bindsym --release button3 exec --no-startup-id import /tmp/latest-screenshot.png
     # execute custom script when scrolling downwards
     bindsym button5 exec ~/.i3/scripts/custom_wheel_down
 }