Support split screen like in iTerm2

[lipis]

[vlad-shatskyi]

Thumb up the issue if you want it too. It does influence the priority.

[vlad-shatskyi]

Implementation plan:

• [x] Decide whether to use CSS grid layout (harder to know element sizes, perhaps will be blocked by https://www.chromestatus.com/feature/5705346022637568) or to manually specify sizes in pixels (kind of reinventing the bike, but will have all the necessary information in the code).
• [x] Add a type Pane = SinglePane | VerticalList | HorizontalList. Splitting would replace a SinglePane with a corresponding list.
• [x] Add Window -> Split Horizontally and Window -> Vertically to the menu.
• [ ] Figure out how to find a pane to the left of the current one in order to switch between them from the keyboard.
• [ ] Add ability to maximize a pane.
• [ ] Make sure alternate screen buffers don't overflow a session container.

[vlad-shatskyi]

You can now split the screen horizontally with cmd + _. Vertical split, resizing and maximizing a pane aren't implemented yet.

Use cmd+j and cmd+k to navigate between splits from the keyboard.

[lipis]

Why not using the same shortcuts as in iTerm? No need to reinvent the wheel and I don't think that they are patented :P

[vlad-shatskyi]

You see, @lipis, it's exactly the shortcuts I use in iTerm. There will be an ability to change them one day.

[denieler]

Sorry, I've not changed shortcut for splitting panes.

[vlad-shatskyi]

Thanks to @denieler vertical splits now work as well! The current shortcut is cmd+| (the button above return, which kind of reminds a vertical split). Configurable shortcuts are coming. https://github.com/shockone/black-screen/issues/441