Add Warning and Check notification type

Similar to an error one, but used to display a Warning or Check (OK) image instead
Add supports for them in UDP interface, service API and mythutil
Display backend is online message as a Check notification.

It's not an error, and should be displayed as such
Add support in default theme for Check and Warning notification.
Don't show airplay connection status during playback.

it quickly gets annoying, especially as any iTunes running like to connect and disconnect to check on the capabilities of the airplay server
Set HTTP Proxy in mythtv-setup and mythavtest

this prevented those applications to work behind a proxy, and mythtv-setup couldn't retrieve the channel list

Fixes #11695
When dropping manually a notification by pressing ESC, remove the "first" one

The first one displayed being either the first notification on the stack, or the last full screen one