In the current scrollback PR the `reset` command does not affect the
scrollback history. To make sure the terminal is properly reset, it
should clear the scrollback history.
This commit fixes this by creating a new and empty grid whenever `reset`
is executed. It takes the current dimensions and history size from the
old grid.
Right now there's an empty ref-test called `grid_reset` without any
content, this should be implemented once #1244 is resolved.
This fixes#1242.
