CC-2977: Never delete files from the database

- Files are marked deleted(file_exists to false) on deletion.
- Dirs are marked removed(removed flag to true) on removal of watched
folder in a usual way. If dir is unmounted, without being removed from
watched list first, it will be marked as not exists(exist flag to false)
- Manage Media Folders will show if dirs exist or not( mounted or not)
- Playlist builder will show if files exists or not

airtime_mvc/build/sql/schema.sql

@@ -42,6 +42,7 @@ CREATE TABLE "cc_music_dirs"
 	"id" serial  NOT NULL,
 	"directory" TEXT,
 	"type" VARCHAR(255),
+	"exists" BOOLEAN default 't',
 	"removed" BOOLEAN default 'f',
 	PRIMARY KEY ("id"),
 	CONSTRAINT "cc_music_dir_unique" UNIQUE ("directory")