safeslinger release 1.6.12

- Separated source for developers into easy exchange library and demo
for any Android application to use (github.com/SafeSlingerProject).
- Improved exchange UI to display better icons and selected values.
- Added setting to customize message notification: sounds, vibration.
- Added setting to change message font size (default is slightly
larger).
- Added setting to auto-decrypt messages (default to true).
- Fixed loss of selected attachment on Compose tab.
- Restored Sling Keys Assistant on Sling Keys tab.
- Stability fixes.