This commit provides the heart of the split functionality.
It adds a couple of key assignments (that are not finalized)
for splitting the active pane horizontally or vertically.
Clicking in a pane will activate it.
Selection is constrained to the active pane.
Resizing a window will grow/shrink the right/bottom side of a split.
The scrollbar applies to the active split.
There's not yet a way to resize a split at all.
There's not yet a key assignment for changing the active split.
The software renderer doesn't know how to render the splits correctly.
refs: https://github.com/wez/wezterm/issues/157
