Issue title: airports and heliports icons
 Issue body: Hello, version 6 17 3 (Ubuntu 21.04 and Windows 10), the airports and heliports icons do not display on the ADSB Modulator plug-in map ??
![image](https://user-images.githubusercontent.com/68333247/141477166-11cfb61b-9bcf-482b-9f64-4ce5dc9822a0.png)

 Comments: 
 Comment 0: on the map feature too
 Comment 1: You have the airport display distance at 1km - that means it will only display airports within 1km of your antenna position - change that to 100km or so.
 Comment 2: Note that the Map feature doesn't currently display Airports - but I'm working on supporting it.
 Comment 3: I just tried 1, 10, 100, 1000 km nothing is displayed, it worked in previous versions (including 1 km), I have an airport 12 km from my home
 Comment 4: Try redownloading the airport database

Check C:\Users\you\AppData\Roaming\f4exb\SDRangel\airportDatabase.csv - it should be about 10MB
 Comment 5: I have done this several times already and I cannot find this airportDatabase.csv file on Ubuntu (locate airportDatabase.csv) and Windows. But I think there is a pb, pressing the icon is very short and fast and I don't think a file is actually downloaded (problem de lien URL ?)
{'source': 'GithubIssues', 'repo_name_with_owner': 'f4exb/sdrangel', 'issue_id': 1051988683, 'repo_stars': 1657.0}