[RFC][gui] fix: view state holds wrong file item list while operating with it

As pointed out by #5742 we have an ancient bug where we do some operations on CGUIMediaWindow's m_guiState while it holds the wrong (old) file item list.

This removes the use of a separate CFileItemList object and the copy part to `m_vecItems`. Instead it use `m_vecItems` directly. It also moves the call of `m_guiState.reset()` to `GetDirectory()` directly after items are loaded and before the first operation on `m_guiState` which is `m_guiState->HideParentDirItems()`.

@jmarshallnz it would be very nice if you could take a look and add a comment if it's fine to remove the separate CFileItemList object and the copy part as I don't know why we don't use `m_vecItems` directly.

@Montellese, @Jalle19 ping