title: Qt checkbox/toolbutton with custom/distinct check/unchecked icons
body: <p>I need to have a checkbox like control where the checked and unchecked states use a custom graphic. I've looked at all the docs for QToolButton and QCheckBox, along with QIcon but couldn't find any combination that does what I want.</p>
  
  <p>I just want to use one icon (pixmap actually) in the unchecked state, and a different one in the checked state.</p>
  
  <p>This feels like it should be easy, but the solution (short of a custom control) is eluding me.</p>
  
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  <p>I've tried using a style sheet as well, and <code>QToolButton:checked</code> with a <code>background-image</code> kind of works but I can't get the layout correct -- it's not positioned/sized as with an icon.</p>