On the V20, you have to go to settings > display > home touch buttons > button combination. In there you will see the options for rearranging and changing your home touch buttons, a very useful feature. That's where you'll see the Qslide button to add to your screen.
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As far as I can tell, you can't officially edit the apps in the window. I initially noticed it doesn't have the calculator, which I used often on my G4. Thanks to another member here that grabbed a copy of the G3's LG brand calculator, I was able to side load that and it then added itself to Qslide.
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Outside of that one addition, I haven't seen a way to actually edit what's in there.