Fixed SceneInspector inheritance window layout glitch.

I don't fully understand what is happening but I think it's because when we use the auto-parenting mechanism for a parent which is already visible, Qt realises the child QWidgets earlier in the process, and then fails to update the layout when we change their properties subsequently. By using `_mainColumn()[:] = rows` we not only avoid this, but also do the deletion of existing rows and addition of new ones in one go, which cuts down on intermediate updates and gives a snappier build.
Fixed SceneInspector update bug.

If an update was required before `_updateFromContext()` was called, `self.__playback` would be `None` and errors would ensue.
Made EventLoop remove idle callbacks immediately after calling.

When an idle callback was to be removed (because it returned False), it wasn't removed immediately, but only after calling all other idle callbacks. This made it impossible to re-add the removed callback from one of those subsequent callbacks (an assertion was fired). The removal is now immediate so the callback can be re-added if necessary.
Added closing of SceneInspector popups when target node is deleted.