terminal: add page handling for terminals

The page-layer is a one-dimensional array of lines. Combined with the
one-dimensional lines, you get a two-dimensional page. However, both
implementations, lines and pages only deal with their own dimension. That
means, lines don't know anything about other lines, and pages don't know
anything about cells.

Apart from pages, this also introduces history objects. A history object
is a scroll-back buffer. As some pages like alt-buffers don't have
histories, we keep them separate.

Pages itself forward all cell-related operations to the related line. Only
line-related operations are directly handled by the page. This is mostly
scrolling and history. To support proper resizing, we also keep a
fill-state just like lines do for cells.

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