Fixed selected episode jumping to the top of the list after playing an item

This commit fixes the issue described at https://github.com/boxeehacks/boxeehack/issues/371

Additionally I've reworked from scratch the code that searches for the last unwatched episode, which is now simpler and faster (since it breaks the loop as soon as the first watched episode is found). I've tested this code with as many corner cases as I could think off and it worked flawlessly.