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.

src/ipc.c

@@ -572,6 +572,8 @@ static void dump_bar_bindings(yajl_gen gen, Barconfig *config) {
         y(integer, current->input_code);
         ystr("command");
         ystr(current->command);
+        ystr("release");
+        y(bool, current->release == B_UPON_KEYRELEASE);
 
         y(map_close);
     }