Sim chooser in paparazzi center

Added a simple popup to choose the simtype and launch via the new [ppzsim-launch] (https://github.com/paparazzi/paparazzi/blob/3e58b39e52b102a2a8205a7f10c45080aac9b6e6/sw/simulator/ppzsim-launch) python script.

This python launcher can launch all three simulators we have and (mostly) makes sure not to pass invalid arguments (e.g. boot, norc).
It's also nice to have one tool with proper help messages, so you know what is available.

If you have a better name than ppzsim-launch, we could change that...
Probably better to not directly replace launchsitl, as that would break existing usage...

I suck at OCaml, so this is only really really basic. A popup box with 3 buttons, one for each simulator type: simple fixedwing sim (ocaml), jsbsim, nps
* Ideally this would be a bit more elaborate chooser, where you can set additional options like flightgear viz, etc...
* It would also be nice if that would be a dropdown menu with the available simulators according to which targets are available for the current aircraft (probably better to show all, but disable ones that are not available)