Add `ui.sanity_check` hook-in

When investigating an issue with a tooltip that was too wide, I added this to Ui temporarily, and started calling it from random places:

    #[inline]
    #[track_caller]
    fn sanity_check(ui: &egui::Ui) {
        if cfg!(debug_assertions)
            && ui.layer_id().order == egui::Order::Tooltip
            && ui.spacing().tooltip_width + 100.0 < ui.max_rect().width()
        {
            panic!("DEBUG ASSERT: Huge tooltip: {}", ui.max_rect().size());
        }
    }

We should allow users to install such a sanity-checks as callback, and then call ui.santity_check(); in misc places in egui (e.g. after adding a widget, or when closing a scope).

The default sanity check could check for NaN:s etc, and then call the user-provided ones.

Should only be enabled in debug builds.

Start by examining the Ui type and the proposed ui.santity_check() call sites, such as after adding a widget or closing a scope. The issue is done when users can install sanity-check callbacks, a default check handles NaN values, callbacks run only in debug builds, and the behavior is covered in the relevant UI paths.