Show warning if powersave mode is/ get's enabled

[dennisguse]

Enabling powersaving mode usually disables the GPS and thus no track is recorded. At least, we should show a warning to the user.

Originally posted by @rafaelmuynarsk in https://github.com/OpenTracksApp/OpenTracks/issues/730#issuecomment-825883208

[ph818]

Would it be possible to show the user a warning if a lot of points are being discarded due to location inaccuracy? Inaccurate location signal and overzealous power saving can both lead to gaps in the recorded track and as a user it's hard to tell what's happening

[dennisguse]

@ph818 good idea! This was not a high priority so far though.

[dennisguse]

Might actually be fixed by: https://github.com/OpenTracksApp/OpenTracks/pull/1114