Download all files (for full offline mode) + changeable video source

Adds a button to the Cache tab in settings allowing the user to download all videos to the cache + count of current downloads
Adds a new video source tab that allows the user to change the video source. This is to allow for people to host their own JSON file with videos if they wish. (can test with https://t27q97zg19.execute-api.us-east-1.amazonaws.com/prod/aerialAltJSON)
Moved away from using the app.config file as it would break if it wasn't in the distribution, and the current instructions didn't include it. Instead added a global variables class

cDima / Aerial