Fix: Fixed ArgumentOutOfRangeException in ItemViewModel

[hishitetsu]

BeginBulkOperation suppresses CollectionChanged and allows changing the collection in non-UI threads. But changing the collection still needs to be done in UI threads, because changes in non-UI threads may not be seen from UI threads soon due to the memory cache.
This PR puts all FilesAndFolders operations into UI threads. This may affect UI performance, but ArgumentOutOfRangeException should no longer be thrown.

Resolved / Related Issues

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  Closes Bug: Adding a file to an open Files directory causes a crash #14169

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[hez2010]

But changing the collection still needs to be done in UI threads

I think only signaling the event is necessary to happen on UI threads.

[hishitetsu]

I think only signaling the event is necessary to happen on UI threads.

I thought so, too. But I found that putting a breakpoint between the collection change in the non-UI thread and the triggering of the event in the UI thread prevents the exception that could have occurred without the breakpoint. This implies that collection changes are not synchronized between threads. In fact, this PR prevents exceptions. I guess ObservableCollection doesn't assume that collection changes and event triggering take place in different threads.

[hishitetsu]

I believe this time there is no problem and this simplifies the process.