[messages] disable message frame sensitivity during drag

- it also disable update of new message
- it can solve a bit of #966
[messages] hide notebook and display help during drag

It is an other way to solve #966 with the advantage that the "drag
route" is made easier since during drag most of the messages window is
hidden
[ground_segment] represent unconfirmed settings with infinity

proposal to fix #1012:
- server: represent initial/unknown/unconfirmed settings with infinity
- page_settings: set label text to "?" upon commit and on update if value is "inf", also set label to "?"
[cockpit] add tooltip for current settings value label
[ground_segment] settings: also set current value lable to ? when requesting update
[ground_segment] use None/Some instead of abusing infinity
[messages] add tooltip to field buttons
[plotter] add a range of curves to plotter if value is an array
Merge pull request #1020 from paparazzi/messages_drag_sensitivity

Messages drag sensitivity
[vision] add computer vision library in standard modules + 2 modules

- video RTP stream
- image download

for ARDrone2
[settings] handle update from other agents than GCS and save based on last known value
Fixed 3-mode switch. Improved file name with respect to strongly recommended mode switch position on gold simulator usb dongle
Merge pull request #1024 from kevindehecker/improved_goldsimulator

Fixed 3-mode switch. Improved gold simulator file name.
Merge pull request #1023 from paparazzi/unconfirmed_settings

- add tooltip for current settings value label
- settings: set current value label to ? while unconfirmed (requesting update, commit)
- represent initial/unknown/unconfirmed settings with `None` in OCaml
- represent initial/unknown/unconfirmed settings with `?` in DL_VALUES on Ivy
- save based on last known value

solves #1012
[telemetry] disabling telemetry made easier

- reorganize telemetry makefiles for FW
- protect some calls and provide default implementation of
  register_periodic
- improve generator with attribute unused keyword
[mavlink] Basic implementation (needs cleanup)
[mavlink] configurable device
[ext] generate mavlink headers
[modules] mavlink: fix time
[modules] mavlink: fix waring comparing to NULL