Previously, the stack would be reset when switching between screens.
With the new search flow this means that search results are lost, e.g. when switching
from search results to the queue and back.
* implement search for shows/podcasts
* create Playable supertype for queue to contain tracks and episodes
* wip: implement playback of episodes
* load spotify id from uri instead of raw id to fix podcast playback
* show duration for podcast episodes
* implement generic status bar for playables (tracks and episodes)
omit saved indicator for now as the library does not yet support podcasts
* instead of only the last 50 fetch all episodes of a show
* refactor: extract Playable code to separate file
* implement playback/queuing of shows + sharing url
* implement podcast library
* migrate mpris code to Playable supertype
