New Control: Split Panel

[tig]

This may already be possible, but I'm missing it:

I want a control that lets me build paneled UIs like UI Catalog more easily:

But with arbitrary horizonal and vertical panels. It's a very common console GUI idiom.

I should be able to resize the panels with either the mouse or keyboard (with well defined shortcut key idioms that would be consistent across all Terminal.Gui apps).

TAB should navigate gracefully between panels.

I should be able to add and remove panels dynamically at runtime.

Panels should have (optional) Titles (like shown above).

Unlike above, the border between panels should be a single column (or row for horizontal panels).

[tig]

We'll need this for Wizard (see #124). But in that case, there's another requirement:

• Borders & titles of the panels should be optional, except for a single border between panes

Like this:

[tig]

Fixed by #2258