The crate containing the entry point is called alacritty, and the crate
containing everything else is called alacritty_terminal.

This resolves issues with copy-pasting tabs by including them in the
pasted string.

Selection of tabs is still inconsistent with what might be expected
based on other terminal emulators, however the behavior hasn't
regressed.

This fixes https://github.com/jwilm/alacritty/issues/219.

Now, only cells that have been used are cleared. This is achieved by
using a "occupied" memo on the Row itself. The value, `occ`, is updated
wherever the Row is accessed mutably, and it's cleared to zero in
Row::reset.

The tests for grid scroll_up and scroll_down were updated to include a
test on the value `occ` and slightly refactored, but are otherwise
equivalent to the previous implementation of those tests.

Because of the change to the `Row` struct, the ref tests were updated so
Deserialization keeps working as expected.

Some tests are still not passing, though.

A migration script was added to migrate serialized grids from
pre-scrollback to the current format. The script is included with this
commit for completeness, posterity, and as an example to be used in the
future.

A few tests in grid/tests.rs were removed due to becoming irrelevant.

Padding can be configured by using the `padding` field in the config
file, like so:

    padding:
      x: 2
      y: 2

which would result in a 2px padding within each side of the window.