Replace `zenity` dependency with `alerta`

[SludgePhD]

(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 Ctrl+C should copy the dialog message to the clipboard SludgePhD/Alerta#3
• No accessibility support (AT-SPI requires speaking DBus) Accessibility SludgePhD/Alerta#4
• No support for selecting buttons with the keyboard Keyboard Navigation SludgePhD/Alerta#2

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

[PolyMeilex]

Hi, I primarily want to target Wayland, but this looks cool nonetheless, could you share cargo tree of the project? (I am on a dependency triming hunt recently 😅)

[SludgePhD]

alerta v0.1.2 (/home/sludge/code/Alerta/crates/alerta)
├── ab_glyph v0.2.32
│   ├── ab_glyph_rasterizer v0.1.10
│   │   └── libm v0.2.15
│   ├── libm v0.2.15
│   └── owned_ttf_parser v0.25.1
│       └── ttf-parser v0.25.1
│           └── core_maths v0.1.1
│               └── libm v0.2.15
├── bytemuck v1.24.0
├── euclid v0.22.11
│   └── num-traits v0.2.19
│       └── libm v0.2.15
├── rapid-qoi v0.6.1
│   └── bytemuck v1.24.0
├── raqote v0.8.5
│   ├── euclid v0.22.11 (*)
│   ├── lyon_geom v1.0.18
│   │   ├── arrayvec v0.7.6
│   │   ├── euclid v0.22.11 (*)
│   │   └── num-traits v0.2.19 (*)
│   ├── sw-composite v0.7.16
│   └── typed-arena v2.0.2
└── x11rb v0.13.2
    ├── gethostname v1.1.0
    │   └── rustix v1.1.2
    │       ├── bitflags v2.10.0
    │       └── linux-raw-sys v0.11.0
    ├── rustix v1.1.2 (*)
    └── x11rb-protocol v0.13.2

Reason it only targets X11 is simply because that's the lowest common denominator, and duplicating all the logic for Wayland would just increase code size unnecessarily and add a bunch of extra work. Everything should work on XWayland.