QML UI: Podcast and episode models as context property
QML UI: Fix download status reporting
QML UI: Decouple Python upcalls from QML UI
QML UI: Fix/refactor episode playback (un-)queueing
QML UI: Decoupled current episode handling
QML UI: Decoupled UI shutdown handling
QML UI: Remove QObjectProxy and "main" from Python
QML UI: Refactoring / grouping of upcalls