Introduce a "Selectable Toggle" widget and use it for the 3D view's camera kind

[abey79]

What

This PR introduces UiExt::selectable_toggle(), which builds on Ui::selectable_{label|value} to make a horizontal, toggle-like widget:

let mut flag = false;
ui.selectable_toggle(|ui| {
    ui.selectable_value(&mut flag, false, "Inactive");
    ui.selectable_value(&mut flag, true, "Active");
});

Now used for the 3D view's camera kind selection:

Also fix a crash in re_ui_example.

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[teh-cmc]

neat

[gavrelina]

Super cool!
I have 3 small visual improvements:

1. the alignment of the text in the box is visually off (not centered optically, see screenshot example, it's just 1-2 px)
2. the font color of the item that is not selected shall not be the same bright, it shall be less contrast, so please use the same shade as you use for 'camera' or 'coordinates' (in your screenshot)
3. the grey is not the grey 😬, it's warmer shade, and we shall use colder one. I propose to use the same grey as you'd use for the secondary label (same as for 'camera' or 'coordinates'). It will especially look better once you change the font color.

[abey79]

For (1), the bad news it that this is a egui issue. The good news is that it's caused by an unaligned cursor coordinate, which I can fix. This makes me want a debug tool for this: emilk/egui#4927

Edit: actually it's not only that. egui seems to have an imperfect text-in-box alignment—not sure how to fix it (font dependant?)

emilk/egui#4929