Rewrite asteroid-btsyncd using Qt5::Bluetooth components

[beroset]

The current code uses no Bluetooth support from Qt, so the purpose of this rewrite is to use those features of Qt that simplify the code and make it easier to maintain. Several contemplated features should be easier to add once this rewrite is complete.

[beroset]

Much of this is done. See https://github.com/beroset/asteroid-btsyncd/tree/comprehensive-rewrite