playqueue: Understand that items of type 'station' are playable

Fixes #1991
keyboard: Map Shift-F2 to previos track and Shift-F3 to next track
skin: Add a list item for 'station' item types
prop: Better source tracking of subscriptions
prop: Fix a bunch of additional problems with prop symlinks
plugin: Rework how prop model is built from plugin repo and installed index

This fixes some memory leaks and some nav issues

Fixes #1980
js: If all else fails, try to extract character encoding from <meta> tags

Fixes #1993
js: Fix possible crash if <meta> tag is corrupt
skin: Make list scrollbar focusable

Also when moving the scrollbar, make sure focused item in list follows on

Fixes #1111