Add attribute "keep" to BLE output

[Jens Noriʇzsɔɥ]

The “keep” attribute is only supported by the current phyphox beta version for 1.2.0 (file format 1.19) while the editor produces output for the release version (file format 1.18). There is no way to set a target version in the online editor and I would not expect that we ever add such complexity to it. Sorry.

At the moment, the attribute would only work on Android with the iOS beta being not yet ready.

(I suppose there are likely a few threads on this “unexpected behaviour” and how to work around it.)