Add utility functions to convert variant values

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Use new variant util functions in KeyValueStorage

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add KeyValueTree

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>