Replace PurpleIdleUiOps with the PurpleIdleUi Interface

This continues our path down replacing the UiOps with Interfaces so that
developers can write user interfaces in other languages using gobject
introspection.

Testing Done:
Ran pidgin3 and set it to auto idle after 1 minute of system idle time. This attempted to query xscreen saver, but something didn't work there, but that's been happening before this change. Not sure if it's my system or the code just needs fixing.

Ran finch3 set idle time to 1 minute and verified that it set me to away after that minute.

I also used the libpurple idle time for both pidgin3 and finch3 and verified they continued to work as well.

Reviewed at https://reviews.imfreedom.org/r/1091/