Multi-grid support

[Lyude]

Eventually, I'd like to have us have full support for using multiple UI grids for a number of reasons

• I have nostalgia for vim-X11…
• This would potentially be a pretty significant performance improvement in situations with lots of split windows open, as GTK+ should be able to handle full NvimViewport redraws per-grid and optimize the dirty regions much more optimally with multiple NvimViewport widgets as opposed to one.
• The potential (I think?) for more fine-tuned control over the size of split windows, and make things nice like Tilix. For example:
  • The ability to temporarily "maximize" a split window without affecting the size of other buffers
  • The ability to detach buffers into separate windows, which would also be a pretty big improvement for multi-monitor setups

[Lyude]

issue #69? nice :fire: :100: