ADS-B: Add altitude filter, alerting on proximity

#### Issue details
Mission Planner displays _all_ ADS-B reporting aircraft - even those flying at 28000 ft...

If operating a UAV/ sUAS at 500 ft AGL or below, though, I only really need to be aware of manned aircraft in a limited layer of airspace. (Mostly I am concerned about interference with EMS/ police helicopters, General Aviation and low-flying military jets.)

Adding a **customizable filter** (e.g. show only aircraft flying at x-y ft/m altitude) would allow the UAV pilot to focus on only those aircraft that he needs to stay clear of.

Adding a **"proximity" alert** - both an audible warning and a visible one (manned airplane on map flashes with a red circle which get's more intense the closer it get's to the UAV) can warn the remote pilot of any aircraft approaching and posing a collision danger at his altitude level.

(This of course is not trivial, as it ideally also needs to take into account the flight route/ trajectory, the altitude vector and the speed, in combination with the UAV's flight plan. A first pointer could maybe be gleaned from the source code of [Virtual Radar Server](http://www.virtualradarserver.co.uk/Download.aspx) > Windows 7 / Vista Proximity Gadget)

Ideally this solution would also incorporate warnings from FLARM/ uAvionix (as hinted in more detail [here](https://discuss.ardupilot.org/t/flarm-pilot-aware-open-glider-network-decoder/23273/7)).

Having a filtering option would certainly be great for starters and de-clutter the GUI of ADS-B sources which are not a factor.
Thanks for considering this.
#### Version
1.3.61

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[X] Plane
[ ] Rover
[ ] Sub

The issue concerns Mission Planner's ADS-B display and proposes altitude filtering, proximity alerts, and possible FLARM/uAvionix warnings, but names no files, tests, or code entry points. Start by locating the ADS-B display implementation and determine which single scope can be specified; completion is not defined because the request combines several substantial features.

Written by the indexing model from the issue text.