[settings] improve settings in pprzcenter

- check box for faster enable/disable
- loading from modules
[settings] add a few more settings in modules
[settings] variable size modules settings area
[settings] add settings directly inside modules files
[settings] allow several groups of settings in modules
[settings] move settings to module files and remove old files
[settings] fix example conf
fix conf_example
remove another unneeded settings file
[settings] select new modules' settings by default
[ocaml] remove leading white space when get tree values
[conf] update all example airframe for new modules' settings
[settings] remove settings from automatically unloaded modules
[modules] fix modules' settings
[ocaml] md5 sum target dependent due to unloaded settings
[imu] imu_scale_x functions
[paparazzicenter] settings area adapt its size (no more scrollbar)
Merge pull request #834 from paparazzi/modules_settings

let's see what's happening then
[radio_control] spektrum for nps/sim

Ocaml sim always uses the radio.xml file and sends ppm values in usec.
So for the time beeing you need to use the spektrum.xml radio file for the sim target if you really want to use the RC in the simple OCaml sim.

closes #503
[conf] update examples using spektrum for sim