Kerbal Space Program Quicksave – How to Undo Loading a Quicksave in KSP?

You can do this with Action Groups.

This youtube tutorial provides a good example of how to do exactly what you want:

In short, once you have the solar panels added to your ship:

1. Click on the "Action Groups" icon at the top of the screen. It's the blue button that looks like two gears.

2. Select "Custom1" or whichever number you would like the action to bind to.

3. Click on the solar panels you would like to extend/retract when the key is pressed.

4. Then select "Toggle Panels" (or "Extend Panels"/"Retract Panels", if you only want this action to work in one direction.)

Now when your ship is launched, you can use whichever number key you bound this action group to to toggle your solar panels.

While you can't undo an end flight (well, at least not without a stringent backup policy), you can prevent yourself from accidentally clicking the end flight button (and then confirming said action) with this mod. Of course, even this isn't fool-safe, as you have to enable the option to begin with. Also, there is the possibility that it will get disabled if you start undocking parts. With my brief testing, it looks like if one piece of a station was set to block end flight, the entire station will block as well.

Previous answer kept for posterity. The game mechanics for ending a flight were drastically changed in version 0.21 such that it's much harder to do accidentally. Yes, it's possible to revert the state of a flight to an earlier point (a quick-save, the launch, or the VAB/SPH), but this is much less destructive compared to ending a flight.

You can still end a flight from the tracking station (by "recovering" vessels that have landed on Kerbin or "terminating" others), but since this is a much more deliberate action, it's less likely that you'll end a critical flight accidentally.