closes#7429
Problem: after a help window was closed, a window was selected and its
autocommands triggered. After that, restore_snapshot was called and the
focused window changed, confusing the user.
Solution: Add function get_snapshot_focus() that returns the window that
holds the cursor in a snapshot. Use this function in win_close to make
sure the right window is selected before any autocommand is triggered.
Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:
1. msgpackparse() will show internal error: hash_add() in case of duplicate
keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
expected. Test was still functioning somehow though. Currently fixed.
