Support floating dialogs in feathers and underlying bevy_ui_widgets.

[gagnus]

Objective

• Many editor requirements require floating dialogs which can be re-ordered and moved around, this would be useful for example for a settings window or entity debugger.

Solution

• I took the existing modal dialog as a starting point and expanded it at the feathers level AND at the underlying bevy_ui_widgets level. The naming was a bit of a clash but on advice I've got for no change to existing stuff:
  • At bevy_ui_widgets level there was modal.rs containing ModalDialog, I've added dialog.rs containing Dialog and also moved RequestClose from modal to dialog.
  • At bevy_feathers level there was dialog.rs containing FeathersDialog (a modal dialog), I've added new FeathersFloatingDialog (a non-modal dialog)

Testing

• Tested locally with a simple app

[alice-i-cecile]

@gagnus CI is slightly mad at you, but what else needs to be done before you think this is ready?

[github-actions]

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