Select all/mark all episode as read

[cm-t 🦄🏂🌈🎵]

Using stable channel on pro5, stable podbird

* List your podcasts
* select one
* long press an unread episode
* you are now in selection mode
* can't find a select all button

It's a regression: not been able to 'select all' in episodes list of a podcast”
(typical usecase: Suscribe a new podcast, want mark all 586 episode as read to spot only the newest)

Note: The icon “mark as read” looks like a select all button, can be missleading, a details to think about if you add a new button for 'select all'

[Nekhelesh Ramananthan (nik90)]

Thanks for taking the time to report this bug. Firstly we need to indeed come up with a "mark as read" icon since it conflicts with the "select all" icon used by many ubuntu touch apps. Otherwise this should be rather easy to fix.