Prompt: Do not call callback in on_enabled

Schedule it for later, when we get "idle". It currently can lead to
crashed because after the callback, the current mode might be different,
leading to a crash when doing the ModeChange hook call.

src/input_handler.cc

@@ -968,8 +968,7 @@ private:
     void on_enabled() override
     {
         display();
-        m_line_changed = false;
-        m_callback(m_line_editor.line(), PromptEvent::Change, context());
+        m_line_changed = true;
 
         if (not (context().flags() & Context::Flags::Transient))
             m_idle_timer.set_next_date(Clock::now() + get_idle_timeout(context()));