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.

i3bar/include/configuration.h

@@ -27,6 +27,7 @@ typedef enum { M_DOCK = 0,
 typedef struct binding_t {
     int input_code;
     char *command;
+    bool release;
 
     TAILQ_ENTRY(binding_t)
     bindings;