Add Modal and Memory::set_modal_layer

[lucasmerlin]

• Closes Modal dialogs #686
• Closes Modal Dialogs #839
• I have followed the instructions in the PR template

#5370 should be merged before this

This adds modals to egui.
This PR

• adds a new Modal struct
• adds Memory::set_modal_layer to limit focus to a layer and above (used by the modal struct, but could also be used by custom modal implementations)
• adds Memory::allows_interaction to check if a layer is behind a modal layer, deprecating Layer::allows_interaction

Current problems:

• When a button is focused before the modal opens, it stays focused and you also can't hit tab to focus the next widget. Seems like focus is "stuck" on that widget until you hit escape. This might be related to Disabled widgets prevent focus from going to next widget #5359 fixed!

Possible future improvements:

• The titlebar from window should be made into a separate widget and added to the modal
• The state whether the modal is open should be stored in egui (optionally), similar to popup and menu. Ideally before this we would refactor popup state to unify popup and menu

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5358-lucas/modal
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.