title: WPF: how to make KeyBinding on a customcontrol work without focus
body: <p>I have two custom controls. The first one consists of a textbox and a popup/dropdown. The other consist of a expander and some content inside the expander.</p>
  
  <p>Most of the time, I will display the second control inside the popup of the first.</p>
  
  <p>The second control should have a hotkey for toggling the expander.</p>
  
  <p>How can I make a keybinding in the second control in such a way that it will work even if the focus is inside the textbox of the first control?</p>