Automatically remove focus from a widget that is no longer in the object tree about fyne HOT 6 OPEN

This actually confused me recently because after switching content the canvas keyboard/shortcut events weren't working as expected, so I had to sprinkle Unfocus() calls all over the place; just to give an example what it does in terms of behaviour. While logical, it's not very obvious. Also not sure where I would look for it in the documentation though, or if it's even in there already.

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Related: #4374

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Perhaps this should happen on container.Remove. It's less expensive than doing it on every refresh. It does mean you need to use container.Remove instead of directly setting container.Objects to get the benefits though

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Sounds like a good intermediate. Then the complete solution would be set up when we fix the gap that manually removing widgets does not Destroy the renderer ether (IIRC).

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Also semi-related: #4374

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Another note - I would think that destroying renderers should be done on a periodic schedule, like a garbage collection/cache cleaning task. A widget that is removed from display ideally should immediately be unfocused, but that doesn't necessarily apply to render deletion - because the widget could be shown again shortly later. Supersonic is actually using a widget-cache system kind of similar to the collection widgets but for large-scale components of the "pages" or screens of the app, so that quick navigation doesn't create too many new widgets/CanvasObjects.

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