refactor!: Rename Checked to Toggled; drop ToggleButton role

[mwcampbell]

ARIA and AT-SPI have both "checked" and "pressed" flags, but they're functionally equivalent; the only difference is that one is for checkboxes while the other is for toggle buttons. I prefer having just one property. AccessKit already had only a "checked" property, which the AT-SPI adapter translated to "pressed" if appropriate. But I think it makes sense to rename our "checked" property to something more generic, since "checked" only makes sense for checkboxes. Here I took some inspiration from UIA, though I went with the more succinct "toggled" rather than UIA's ToggleState.

I also think it makes sense to not have a dedicated ToggleButton role. Instead, now a Button becomes a toggle button if the toggled property is set. This is the same way that buttons become toggle buttons in ARIA.

If it seems that I'm being inconsistent about whether to add dedicated roles (as I did in a previous refactor) or use state flags to modify the role (as I'm doing here), the guiding principle is this: eliminate as many invalid combinations of role and state as we can. When I previously eliminated state flags and added more roles instead, the only purpose of the state flag was to modify one specific role. But here, where we have a state property that can be false, true, or unset (unlike the simple flags I previously eliminated), the property serves to both modify the role (two different roles in this case) and indicate some state. The goal is the same, by eliminating the ToggleButton role, we make it impossible to mess up the accessibility node by having a ToggleButton with no toggle state, and now, having a plain Button with toggle state is the one way to do a toggle button.

[DataTriny]

I totally agree. Someone will have to revert slint-ui/slint#5033 but that's fine.