feat: smart open should prefer alternative window

[Bekaboo]

Currently smart open will pick the FIRST window that is "valid" (a window that is not floating and contains a normal buffer, etc.), which can be unintuitive sometimes.

Suppose I have the following window layout and my cursor is at window B, and in C runs ranger in a terminal:

+-------------------+
|    A    |    B    |
|         |         |
|-------------------|
|    C (terminal)   |
|                   |
+-------------------+

If I switch from B to C using <C-w>j, then open a file from ranger, the current implementation of smart open will open the file in A, but I expect it to be opened in B.

This PR fixes this issue by picking the alternative window (B) if it is valid.

[willothy]

Lgtm, thanks! I've been wanting to update the smart open feature :)