fix: reuse windows and buffers when possible, prevents flashing when switching

[cseickel]

fixes #713

This adds logic to reuse Neotree windows between sources instead of always closing the window when opening a new source. It also improves upon the concept of reusing the buffer when possible instead of recreating it from scratch each time. These are primarily optimizations for quickly switching between sources when they are shown as sidebars.

[cseickel]

@nhat-vo I think this fixes the flashing window problem when switching sources.

[andrevmatos]

I think this may have broken Neotree on different tabs.
Now they aren't independent anymore, and if I leave a tree open on tab 1, switch to tab 2 and try to open Neotree from there, it'll focus tab 1 and its Neotree, instead of opening a new, independent one in tab 2.

[nhat-vo]

@cseickel This looks great, thanks a lot! Sorry, I haven't got much time in the last few weeks. I will have a look at the issue with the different tabs