This removes all CI builds from travis-ci, due to their recent changes
in policy and harsh limitations on builds. With build times over 2
hours, it was a significant hindrance to development.

Instead of Travis CI, the CI is now split on Sourcehut and GitHub. Since
Sourcehut only supports Linux/BSD, all builds on those operating systems
are executed there. The GitHub Actions CI is used to build for
Windows/macOS, which are not available on Sourcehut.

Since asset deployment for releases requires builds on all platforms,
this is also done on GitHub actions. Though the new `upload_asset.sh`
script makes sure that migration in the future is fairly simple and we
do not tie ourselves to the overly complicated GitHub Actions ecosystem.

This should provide more clear feedback whether rustfmt
job is failed or not.

Fixes #3628.

Follow free desktop file naming conventions.

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

This fixes #2273.

This adds the new Alacritty icon to the macOS binary available in the
github releases. On sizes 128x128 and up the image with scanlines is
included.

The ci deployment process should now also offer the svg file for
download so the .desktop file can be used without having to look through
the repos for the icon.