Add adaptive icon for API 26+ and Play Store icon
This branch adds an adaptive launcher icon for API 26+ (guidelines) and an icon for use in the Play Store (guidelines).
This prevents the logo from appearing as a drop-shadowed circle within a drop-shadowed roundrect on API 26+ devices and in the Play Store. Its appearance on other devices should be unaffected (I tested API 17 and 23).
Fixes #1456 (closed).