Add support for modal windows to re_ui and use it for the Space View entity picker

[abey79]

What

Since egui doesn't have support for modal windows yet (emilk/egui#686), we rolled our own for the Space View entity picker ("Add/remove entities"). This PR abstracts the modal creation code into a dedicated feature in re_ui, and use that abstraction for the space view entity picker.

The new re_ui::modal has support for two aspects of modal window management:

• The modal window itself (re_ui::modal::Modal), which handles dimming the background, a close button, and exiting with ESC and/or clicking outside of the window.
• The handling of the modal window (re_ui::modal::ModalHandler). The Modal struct should be kept only while the modal is actually shown, so it's typically held in and Option<Modal>. ModalHandler implements the house keeping of populating the Option when the modal must be displayed, and dropping it when the modal is closed.

Note: with this PR, the modals are now properly centred, although erroneous centring behaviour might happen that require deleting your app.ron. This will eventually be fixed with a new egui release that includes emilk/egui#3721

Usage:

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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[Wumpf]

was hoping this also fixes the bug of modal windows being off-center. Alas it does not

[Wumpf]

Looks whatever prevented pinning the dialog to the middle got fixed \o/ - plz have a look at d9bef9d

[Wumpf]

beautiful!

[abey79]

Looks whatever prevented pinning the dialog to the middle got fixed \o/ - plz have a look at d9bef9d

This is still not working for me, ie. it centres the top-left corner instead of the center of the window as specified by the pivot function.

Edit: I've been it by emilk/egui#3721