Refactor play_or_download() progress tab action code.

Refactor play_or_download() progress tab action code.

Fixes an issue with the toolbar cancel button turning off and on when
entering the progress tab, even though it can't cancel anything until a
selection is made.

Properly cancels failed tasks manually removed from progress tab. Not
cancelling would leave the error icon and prevent downloading or
cancelling.

The download, pause and cancel actions in toolbar and Episodes
menu can now be used to control downloads in the progress tab. The
delete menu item in Episodes menu removes the download from list. This
also allows keyboard accelerators to be used, such as the Delete key
for removing tasks. Accelerators for cancel, and maybe download/pause,
should added in a future PR.

The progress tab context menu now has the same ordering as the other
menus.