Fix split/netrw window close handling

[rbong]

There were window closing handling errors introduced with #22.

When opening netrw the folder buffer is deleted just before opening neovim. This prevents vifm from being reopened when exiting, but as an unintended side-effect it closes the current window. This has been fixed.

When exiting from a non-split window, the current buffer is deleted and the window is closed. Now, the window is closed only if vifm was opened in a split, the window is closed.

This fixes #25

[rbong]

I'm not sure why changes that have already been merged are showing in the diff. I will clean up my history and that should fix it.

[rbong]

Fixed the weird diff issues, should now be ready to merge if you're happy with it

[rbong]

I realized that this and some other changes break switching back to the alternate buffer. I've changed it so that when closing Vifm it will return to the original buffer if appropriate.

[rbong]

It's clear I need to really carefully test this. I have a resolution I believe. It was a careful mix of all of my previous tweaks.

I've performed all of the following tests. If there was a mistake I fixed it and repeated all of the tests.

All of the following settings were tested:

• Split off
• Split on
• netRW replacement

For all of the settings, all of the following things were tested:

• In a single window with no buffer, vifm can be opened and closed without errors or exiting vim
• In a single window with no buffer, vifm can open files without issue
• In a single window with an alt file, vifm can be opened and closed without overwriting the alt file
• In a single window with an alt file, vifm can open new files without issue
• In multiple windows with no buffer, vifm can be opened and closed without closing the window
• In multiple windows with no buffer, vifm can open files without issue
• In multiple windows with an alt file, vifm can be opened and closed without overwriting the alt file or closing the window
• In multiple windows with an alt file, vifm can open new files without issue

Additional tests:

• Split on
  • The split does not persist
• netRW replacemenet
  • Folder does not persist
  • Can open and exit without issue using "e ." (same tests as above)
  • Can open and exit without issue using "vs ." (same tests as above)

All tests were repeated in vim and neovim.

I have also cleaned up my branch history.

Unrelated issues encountered:

• Cannot open a folder from netRW in Vim because the exit function is already in use, will fix in a separate PR since it has little to do with window handling

[xaizek]

Great. Thanks a lot, @rbong!