WIP: Add sailfishapp code from Whisperfish

This commit does also contain a minimal app example that can be build by using the sfdk tool provided by Jollas ApplicationSDK.

TODOs:

• support i486 target
• support aarch64 target
• re-enable detect_qt_version in build.rs
• setup CI
• add build info to README
• check if the prep and build stage do work in VM, too
• (optional) use cargo workspace