Implemented settings for warnings tts items
fixes bug from PR #1008
400 ft exceeded warning
implemented tts arming failures and renamed failsafe to warning
implemented tts setting for lost signal
extracted 121.19 to constant, moved settings to droidplannerprefs
use context.getstring(), implemented weak signal warning
implemented low batery warning
removed <15% battery and replaced with autopilot-reported low battery
hiding solar and wind activity checkboxes
Merge pull request #1012 from TShapinsky/issue_835_speech_warnings

Issue 835 speech warnings