Persistent last modified

This sets _last_modified to the latest modified time of active pushes.
This fixes videos playing that were pushed while Kodi was closed. Previously, after Kodi was opened, when a push was received, all pushes not previously dismissed would interrupt each other.

Also added a setting to prevent pushed videos from interrupting playing video.

Ignore the weird commits. I accidentally added some code while still on the develop branch and had some adventures getting the code merged into this branch :)

Let me know if you see anything wrong here.