Accept the zoom in hotkey when shift is held

[karpovantonme]

Problem

ctrl + + does not zoom in on layouts where + is typed as shift + =. That covers US and UK ANSI and most others. On layouts where + has its own key (Italian, German) it works, which is probably why it went unnoticed.

The subscription in src/gui/sniffer.rs matches the whole modifier set by equality:

Keyboard(Event::KeyPressed { key, modifiers, .. }) => match modifiers {
    Modifiers::COMMAND => match key.as_ref() {
        ...
        Key::Character("-") => Some(Message::ScaleFactorShortcut(false)),
        Key::Character("+") => Some(Message::ScaleFactorShortcut(true)),

Modifiers is a bitflags struct, so Modifiers::COMMAND as a pattern means "command and nothing else". To produce the logical key + the user has to hold shift, and iced_winit::conversion::modifiers passes shift straight through:

result.set(keyboard::Modifiers::SHIFT, modifiers.shift_key());

So the event arrives with COMMAND | SHIFT, matches neither Modifiers::COMMAND nor Modifiers::SHIFT, and falls into the catch-all. Zoom out is unaffected because - needs no shift, which is why only one half of the shortcut is broken.

The shortcut came from #554, where it was asked for as an accessibility feature, so it seemed worth fixing properly.

Change

• accept command + shift + +
• accept = as a zoom in key too, without shift. That is what browsers do, and it gives people a shift-free way in
• the key mapping moves into hotkey_message(), so it can be tested. Behaviour outside the zoom keys is unchanged, I kept the match arms in the same order

Checks

• cargo test - 178 passed, 0 failed
• cargo fmt --check - clean
• cargo clippy --all-targets - 57 warnings before the change and 57 after
• test_zoom_hotkeys fails on main with the fix reverted and passes with it, so it guards the actual bug

If you would rather keep the subscription as it was, the one-line version is to add "=" next to "+" in the existing arm. That fixes the shift-free path but leaves shift + = dead. Happy to cut it down if you prefer.