Prompt event style changes and cleanup
Prompt event style changes and cleanup

* Rename "ExtraChoice" to "PromptChoice", and add an attribute for the short
letter used by the choice (short).
* Add TerminalImportSession._get_plugin_options() for isolating the sending
of the event and the processing of the returned values (flattening, check for
letter conflicts).
* Remove unnecessary TODO lines and other style cleanups.

madmouser1/beets,MyTunesFreeMusic/privacy-policy,MyTunesFreeMusic/privacy-policy,Freso/beets,beetbox/beets,shamangeorge/beets,madmouser1/beets,xsteadfastx/beets,ibmibmibm/beets,diego-plan9/beets,pkess/beets,Kraymer/beets,madmouser1/beets,parapente/beets,diego-plan9/beets,lengtche/beets,pkess/beets,swt30/beets,beetbox/beets,lengtche/beets,mried/beets,SusannaMaria/beets,parapente/beets,mried/beets,artemutin/beets,mosesfistos1/beetbox,jcoady9/beets,lengtche/beets,Kraymer/beets,xsteadfastx/beets,jackwilsdon/beets,shamangeorge/beets,diego-plan9/beets,xsteadfastx/beets,Freso/beets,sampsyo/beets,swt30/beets,pkess/beets,sampsyo/beets,mried/beets,sampsyo/beets,SusannaMaria/beets,ibmibmibm/beets,mried/beets,artemutin/beets,Freso/beets,artemutin/beets,parapente/beets,Kraymer/beets,LordSputnik/beets,jackwilsdon/beets,beetbox/beets,madmouser1/beets,beetbox/beets,artemutin/beets,jcoady9/beets,mosesfistos1/beetbox,swt30/beets,mosesfistos1/beetbox,Kraymer/beets,ibmibmibm/beets,mosesfistos1/beetbox,xsteadfastx/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,SusannaMaria/beets,jackwilsdon/beets,sampsyo/beets,LordSputnik/beets,jcoady9/beets,shamangeorge/beets,MyTunesFreeMusic/privacy-policy,LordSputnik/beets,LordSputnik/beets,lengtche/beets,jcoady9/beets,swt30/beets,SusannaMaria/beets,jackwilsdon/beets,parapente/beets,Freso/beets,diego-plan9/beets,ibmibmibm/beets,shamangeorge/beets