goto: use current window; respect / use 'switchbuf' setting in general

I find it irritating, when "goto" goes to another/existing window (in another tab), via `_newtab`.

I think that jedi-vim should use Vim's `switchbuf` setting in general:

```
'switchbuf' 'swb' string (default "") global {not in Vi}

This option controls the behavior when switching between buffers.
Possible values (comma separated list):

useopen If included, jump to the first open window that
contains the specified buffer (if there is one).
Otherwise: Do not examine other windows.
This setting is checked with |quickfix| commands, when
jumping to errors (":cc", ":cn", "cp", etc.). It is
also used in all buffer related split commands, for
example ":sbuffer", ":sbnext", or ":sbrewind".

usetab Like "useopen", but also consider windows in other tab
pages.

split If included, split the current window before loading
a buffer for a |quickfix| command that display errors.
Otherwise: do not split, use current window.

newtab Like "split", but open a new tab page. Overrules
"split" when both are present.
```

Using `switchbuf` could eliminate some of the settings (`g:jedi#use_tabs_not_buffers` and `g:jedi#use_splits_not_buffers`), or rather could derive their defaults from it (`g:jedi#use_splits_not_buffers` allows for providing `left`, `right` etc.)

I find `jedi#use_tabs_not_buffers` (enabled by default) an odd setting by itself: e.g. for the quickfix list for the usages, there should be no extra handling, just use the standard quickfix mechanisms (which uses `switchbuf`).

Apart from that, `goto` should just use the window where it was called from, at least by default.

Some concrete questions:
- should "rename" open new buffers? Probably yes, because these files have been changed.
- `:Pyimport` should behave like "goto", i.e. not open a new buffer by default (according to `switchbuf`)?