Support infoview auto-close

[rish987]

Currently, :quitting the last Lean buffer in a tab leaves the infoview dangling in a detached state. Instead, it should automatically close the associated infoview, and :close should leave it in a detached state.

This will likely involve including the set of associated windows as part of the Infoview object, adding/removing these windows on Lean file BufEnter/BufExit, and once the last such buffer is exited, this will trigger the infoview to close as well. We should also make it (dynamically) configurable per-infoview whether or not this autoclose happens, in case someone doesn't like this -- that is, some kind of pin function that keeps the infoview around even when no Lean buffers are open.

[Julian]

What is the user-facing functionality that this is trying to support? Is it not just #84?

The second part of this sounds like #43, or is it trying to say something different?

[rish987]

Ah yes this is a dupe of #43, my bad!

[Julian]

All good just making sure I wasn't missing something!