racket/gui: fix `get-key-release-code' result for a key-press event

Return 'press (matching docs and pre-v5.1) insteda of 'down.
Add ".desktop" file support

A launcher can have a ".desktop" file (found like other files: as the
same name as the main launcher file, but with a ".desktop" suffix),
where the "Exec" and "Icon" fields are added automatically. A ".png"
or ".ico" file can be supplied for the icon (where the ".ico" file
is already used for Windows launchers).

Closes PR 13953

Fix various problems with Unix-style install from an installer.

Also, add an ugly icon for the Racket Package Manager, with the hope
that it will provoke someone to create a nicer one.