Identify a pane by its window and its pane index

By identifying a pane by both its window and pane index,
VimuxCloseRunner() will always close the correct pane, even in case
VimuxCloseRunner() is called from a Vim instance in a different window
than the current window.
Merge pull request #97 from ton/fix-closing-incorrect-pane

Identify a pane by its window and its pane index