Improve startup sequence termination conditions

If a window with _NET_STARTUP_ID set is moved to another workspace, it
will delete any associated startup sequence immediately. This will also
occur if a window has a leader with _NET_STARTUP_ID set, if the leader
has no container (never been mapped).

A startup sequence may also be deleted if it's matched by
startup_workspace_for_window() and its 30-second timeout has elapsed.

include/startup.h

@@ -31,12 +31,27 @@
  */
 void start_application(const char *command, bool no_startup_id);
 
+/**
+ * Deletes a startup sequence, ignoring whether its timeout has elapsed.
+ * Useful when e.g. a window is moved between workspaces and its children
+ * shouldn't spawn on the original workspace.
+ *
+ */
+void startup_sequence_delete(struct Startup_Sequence *sequence);
+
 /**
  * Called by libstartup-notification when something happens
  *
  */
 void startup_monitor_event(SnMonitorEvent *event, void *userdata);
 
+/**
+ * Gets the stored startup sequence for the _NET_STARTUP_ID of a given window.
+ *
+ */
+struct Startup_Sequence *startup_sequence_get(i3Window *cwindow,
+    xcb_get_property_reply_t *startup_id_reply, bool ignore_mapped_leader);
+
 /**
  * Checks if the given window belongs to a startup notification by checking if
  * the _NET_STARTUP_ID property is set on the window (or on its leader, if it’s