Replace `zenity` dependency with `alerta`

(hope you don't mind the self-advertisement)

On Unix, rfd currently relies on zenity being installed in order to create message boxes. This isn't great, since it's a runtime dependency that can't be compiled into the program, and it might not always be installed.

Alerta is my attempt at displaying simple message boxes on X11 without any C dependencies. It may be worth switching to it to avoid the problems caused by zenity.

This would cause a few regressions, so it might not be acceptable:

• No support for selecting text, or for copying it out of the message box https://github.com/SludgePhD/Alerta/issues/3
• No accessibility support (AT-SPI requires speaking DBus) https://github.com/SludgePhD/Alerta/issues/4
• No support for selecting buttons with the keyboard https://github.com/SludgePhD/Alerta/issues/2

Apart from the accessibility issue, these should be resolvable with relative ease if desired.

Start by locating rfd's Unix message-box path and its zenity integration, then review Alerta's X11 behavior and the listed regression concerns. Determine whether the dependency replacement is acceptable, and verify that message boxes still work on Unix without zenity while documenting or testing the affected text-selection, accessibility, and keyboard-button behavior.