This continues #8202 by fixing two of the known issues: `goto_split` key
binds work and closing a split moves focus to the proper place.
A big improvement in this PR is that for the first time ever in our GTK
backend, the up/down/left/right `goto_split` bindings **use spatial
navigation.** "Spatial navigation" means that the direction to move
focus is done based on the nearest split _visually_ from the current
split, rather than via a tree traversal. We did this on macOS a couple
months ago, with a lot more details there: #7523
Similar to macOS, the spatial navigation is currently based on top-left
corner. Now that our split tree is implemented in Zig though it should
be a lot easier for us to work in the current cursor position as the
reference point.
~~🚧 TODO: Going to add some unit tests for the spatial navigation before
merge.~~
a21b447c75