Option to download enclosed files using DownloadManager

See https://github.com/HoloAddict/FeedEx/issues/72

I've used this version for a few days now and I find the download option very useful for podcasts in my personal use (using other apps while listening to 40+ minute audio files). The new dialog is implemented in commit e5cf0dc . There are a couple of later commits that are just minor changes to the CSS of EntryView.